Methods and apparatus for locating rfid tags

ABSTRACT

A radio frequency identification (RFID) system includes an array of antennas to distinguish line-of-sight (LOS) paths from non-line-of-sight (NLOS) paths. The distance between adjacent antennas in the array of antennas is less than half the wavelength of the radio frequency (RF) signal of the system. Each antenna in the antenna array is also digitally controlled to change relative phase difference among the antennas, thereby allowing digital steering of the array of antennas across angles of arrival (AOAs) between 0 and π. The digital steering generates a plot of signal amplitudes as a function of AOAs. LOS paths are distinguished from NLOS paths based on the shapes (e.g., depth, gradient, etc.) of local extremes (e.g., maxima or minima) in the plot.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.17/199,308, filed on Mar. 11, 2021, which is a continuation of U.S.application Ser. No. 16/717,249, filed on Dec. 17, 2019, which is acontinuation of U.S. application Ser. No. 16/579,949, filed on Sep. 24,2019, which is a bypass continuation of International Application No.PCT/US2018/024950, filed on Mar. 28, 2018, which in turn claimspriority, under 35 U.S.C. § 119(e), to U.S. application Ser. No.62/577,530, filed on Oct. 26, 2017, and to U.S. Application No.62/477,796, filed on Mar. 28, 2017. Each of these applications isincorporated herein by reference in its entirety.

BACKGROUND

Radio Frequency Identification (RFID) technologies have applications inmany commercial areas, such as access control, animal tracking,security, and toll collection. A typical RFID system includes a tag(also referred to as a transponder) and a reader (also referred to as aninterrogator). The reader includes an antenna to transmit radiofrequency (RF) signals as well as to receive RF signals reflected oremitted by the tag. The tag can also include an antenna and anapplication-specific integrated circuit (ASIC) or microchip. A uniqueelectronic product code can be assigned to the tag to distinguish itfrom other tags.

An RFID system can use either an active tag or a passive tag. An activetag contains a transmitter to emit RF signals to the reader and a powersource (e.g., a battery) to power the transmitter. In contrast, apassive tag does not contain a power source and draws power generated bya reader via induced current in the tag's antenna. In a passive RFIDsystem, the reader sends a signal using the reader antenna to excite thetag antenna. Once the tag is powered on (excited), the tag sends thestored data back to the reader.

Signals emitted or reflected by the tag can reach the reader via morethan one path. For example, the signals can travel along a straight line(referred to as the line-of-sight path or LOS path) from the tag to thereader. The signals may also be reflected or scattered off obstructions(e.g., walls and other objects distributed throughout the environment)before reaching the reader. These paths are referred to asnon-line-of-sight (NLOS) paths. In some cases, a given signal may takemultiple paths to the receiver, causing several copies of the signal toarrive at the receiver. The reader perceives each copy of the signal asoriginating from a different direction, or angle of arrival. Thisphenomenon is referred to in the art of RFID technology as “multipath.”

Multipath can cause undesired interference and ghosting. If thedifferent copies of the signals overlap temporally, they may interferewith each other. Destructive interference causes fading. If thedifferent copies of the signals do not overlap with each other,subsequent copies can appear as “ghosts.” These ghosts may deceive thereceiver into determining that extra RFID tags are present.

SUMMARY

Embodiments of the present technology include methods and systems forlocating radio-frequency identification (RFID) tags. One exampleincludes using a system with one or more antennas or RFID tag readers toreceive, from a first RFID tag at a first unknown location, a pluralityof first RFID signals. A processor coupled to the antenna(s) designatesthe first RFID tag as a first virtual reference tag based on theplurality of first RFID signals. The antenna receives at least onesecond RFID signal from a second RFID tag at a second unknown location.And the processor determines a position of the first RFID tag withrespect to the first virtual reference tag based on the at least onesecond RFID signal.

Another example of the present technology uses a first antenna receive afirst line-of-sight (LOS) signal from an RFID tag. A processor coupledto the first antenna estimates a first angle-of-arrival, a first phasedifference, and a first frequency difference of the first LOS signal anddetermines a change in the first phase difference with respect to thefirst frequency difference. A second antenna receives a secondline-of-sight (LOS) signal from the RFID tag. The processor estimates asecond angle-of-arrival, a second phase difference, and a secondfrequency difference of the second LOS signal and determines a change inthe second phase difference with respect to the second frequencydifference. Then the processor estimates a location of the RFID tagbased on the first angle-of-arrival, the change in the first phasedifference with respect to the first frequency difference, the secondangle-of-arrival, and the change in the second phase difference withrespect to the second frequency difference.

Yet another example involves receiving, with a plurality of antennas, atleast one RFID signal from at least one reference RFID tag. A processoroperably coupled to the antennas determines an estimated location of thereference RFID tag based on the RFID signal. It performs a comparison ofthe estimated location of the reference RFID tag to an actual locationof the reference RFID tag. The processor is calibrated based on thecomparison of the estimated location of the reference RFID tag to theactual location of the reference RFID tag. The antennas receives atleast one RFID signal from an RFID tag at an unknown location. And theprocessor determines an estimated location of the RFID tag based on theRFID signal.

Still another example includes receiving, with a plurality of antennas,reference RFID signals from respective reference RFID tags that are atrespective known locations. The antennas also receive at least one RFIDsignal from an RFID tag at an unknown location. A processor coupled tothe antennas determine a location of the RFID tag based on the RFIDsignal and the reference RFID signals.

Still yet another example includes using an antenna array to receive areference RFID signal from at least one reference RFID tag. A processordetermines a receptivity pattern of the antenna array based on thereference RFID signal. The antenna array receives an RFID signal from anRFID tag in an unknown location, and the processor determines a locationof the RFID tag based on the RFID signal and the receptivity pattern ofthe antenna array.

Another example of the present technology includes monitoring a RFID tagby receiving, with at least one antenna, a plurality of RFID signalsfrom the RFID tag over a period of time. A processor coupled to theantenna estimates a plurality of possible trajectories of the RFID tagover the period of time based on the plurality of RFID signals. Theprocessor then identifies a first trajectory in the plurality ofpossible trajectories as corresponding to a line-of-sight (LOS) pathbetween the antenna and the RFID tag.

Another example method of locating an RFID tag includes receiving, witha plurality of antennas, a signal from an RFID tag. A processorgenerates a first digital representation of the response as detected bya first antenna in the plurality of antennas and a second digitalrepresentation of the response as detected by a second antenna in theplurality of antennas. The processor generates a plurality of sums ofthe first digital representation and the second digital representation.Each of these sums is at a relative phase difference representing adifferent angle of arrival for the signal from the RFID tag. Theprocessor uses these sums to estimate a location of the RFID tag.

Embodiments of the present invention include apparatus, systems, andmethods for locating radio-frequency identification (RFID) tags. In oneexample, a method of locating an RFID tag includes sensing, with aplurality of antennas, a signal from an RFID tag to the transmitter. Oneor more analog-to-digital converters (ADCs) generates a first digitalrepresentation of the response as detected by a first antenna in theplurality of antennas and a second digital representation of theresponse as detected by a second antenna in the plurality of antennas. Aprocessor coupled to the ADC(s) generates plurality of sums of the firstdigital representation and the second digital representation. Each sumin the plurality of sums is at a relative phase difference representinga different angle of arrival for the signal from the RFID tag. Themethod also includes estimating a location of the RFID tag based on theplurality of sums.

All combinations of the foregoing concepts and additional conceptsdiscussed in greater detail below (provided such concepts are notmutually inconsistent) are contemplated as being part of the inventivesubject matter disclosed herein. In particular, all combinations ofclaimed subject matter appearing at the end of this disclosure arecontemplated as being part of the inventive subject matter disclosedherein. The terminology explicitly employed herein that also may appearin any disclosure incorporated by reference should be accorded a meaningmost consistent with the particular concepts disclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The skilled artisan will understand that the drawings primarily are forillustrative purposes and are not intended to limit the scope of theinventive subject matter described herein. The drawings are notnecessarily to scale; in some instances, various aspects of theinventive subject matter disclosed herein may be shown exaggerated orenlarged in the drawings to facilitate an understanding of differentfeatures. In the drawings, like reference characters generally refer tolike features (e.g., functionally similar and/or structurally similarelements).

FIG. 1A shows an example system for locating a radio-frequencyidentification (RFID) tag in an environment with both line-of-sight(LOS) and non-line-of-sight (NLOS) paths between the RFID tag and thereceivers.

FIG. 1B shows an example system for estimating an angle of arrival (AOA)of an incident signal.

FIG. 1C is a plot showing an example of a composite signal amplitudeversus angle of arrival/phase difference before (solid trace) and after(rays) de-convolution, or other correction, with the antenna receptivitypattern.

FIG. 1D shows plots of RFID tag signal amplitudes versus angle andelevation for four different RFID tags, each of which is at a differentAOA with respect to the antenna.

FIG. 2 is a block diagram of transmitters and receivers suitable for usein the systems of FIG. 1A.

FIG. 3A is a flow diagram that illustrates a method of locating an RFIDtag using a system like the one shown in FIG. 1A.

FIG. 3B illustrates locating an RFID tag using virtual reference RFIDtags, reference RFID tags, and measurements with multiple readers fromdifferent AOAs.

FIGS. 3C-3F are frames of a video showing the measured location of anRFID tag, indicated by a circle, and a bounding box drawn around anobject tagged with the RFID tag using a neural network or other computervision technique.

FIG. 3G is a flowchart illustrate a method for correlating RFID tagmeasurements with image data.

FIG. 4 illustrates LOS and NLOS signal paths from a moving RFID tag to apair of antennas and the corresponding real and “ghost” velocity vectorsand trajectories derived by the LOS and NLOS signals.

FIG. 5A shows an RFID tag location system in a retail store and astockroom.

FIG. 5B shows RFID tag transmitters and receivers above items for salein the retail store of FIG. 5A.

FIG. 6 shows a graphical user interface (GUI) of a smartphone or tabletdisplaying employee and product locations derived from RFID tag locationdata.

FIGS. 7A-7D show how the GUI can be used to select products or otheritems with RFID tags for specific actions,

FIG. 8 shows how the GUI can display real-time and/or historical motionof an RFID tag in a store, stockroom, warehouse, or other environmentmonitored by an RFID tag location system.

FIGS. 9A-9D show how the GUI can be used to plan and track a pick path,e.g., in a stockroom or warehouse, for pulling items from a pick listbased on RFID tag location data.

FIG. 10 shows how the GUI can be used to identify and locate stray itemswith RFID tags.

FIG. 11 shows how the GUI can be used to fulfill stock requests of itemswith RFID tags.

FIG. 12 shows how the GUI can be used to show location(s) of a selectedproduct tagged with an RFID tag on the sales floor and/or in astockroom.

DETAILED DESCRIPTION

Until now RFID location technology hasn't lived up to the hype. Combinedwith computer vision technology, the inventive RFID location technologyoffers unprecedented speed and accuracy. In fact, it can be over 300times more precise than conventional RFID location technology. Forinstance, the systems and methods disclosed below can be used to locateRFID tags to within 50 cm, 40 cm, 30 cm, 25 cm, 20 cm, 15 cm, 10 cm, 5cm, or 2.5 cm of their actual locations. With this speed and accuracy,it can be used to track RFID-tagged items in real-time, down to theslightest movement. This level of speed and precision makes it possibleto find and restock items nearly instantly and to track interactionsamong RFID tags. For RFID tags on products in a store, this yields dataabout customer interactions with products on an item-by-item level andenables autonomous checkout.

Except where physically incompatible, all of the techniques disclosedherein can be used with each other. For instance, an RFID tag locationsystem can used multiple RFID tag readers to interrogate reference tags,virtual reference tags, and RFID tags from many angles of arrival andcreate (multipath) signatures based on the received signals. Such asystem can locate the tags in two or three dimensions with respect toeach other and/or absolute (known) locations. The resulting locationscan be correlated with video data for training neural networks ormanaging operations of a store or warehouse. The location informationcan also be displayed on a smartphone, tablet, or other device forinventory and supply chain management, etc., as described in greaterdetail below.

1 Multipath and RFID Signals

To address the multipath issues in known radio frequency identification(RFID) technologies and accurately locate an RFID tag, systems, methods,and apparatus described herein employ an array of antennas todistinguish RF signals travelling along line-of-sight (LOS) paths fromRF signals travelling along non-line-of-sight (NLOS) paths. The distancebetween adjacent antennas in the array of antennas may be less than halfthe wavelength of the radio frequency (RF) signal of the system. Eachantenna in the antenna array is also digitally controlled to change itsrelative phase difference with respect to the other antennas in theantenna array. Each distinct phase setting of the antenna arraycorresponds to a distinct angle of arrival (AOA) measured by the antennaarray. As long as the array comprises three or more antennas, thisenables the antenna array to be digitally steered across elevation AOAsbetween 0 and π (i.e., between 0 and 180 degrees) and azimuth AOAsbetween 0 and 2π (i.e., between 0 and 360 degrees).

The digital steering, in turn, makes it possible to generate a plot orother representation of signal amplitudes as a function of AOAs. LOSpaths are distinguished from NLOS paths based on the local extremes(e.g., maxima or minima) in the plot. For instance, the highest (lower),steepest maxima (minima) may be at the AOA corresponding to the LOSpath. Triangulating with the AOAs for two or more distinct LOS pathsyields the RFID tag's location in three dimensions (3D). Theoretically,this approach can locate items to perfect precision under perfectenvironmental conditions. Under realistic indoor conditions, thelocation accuracy may be better than 50 cm using this technique.

The LOS paths estimated above can be used to determine the location ofan RF tag via triangulation. A first antenna or group of antennas isused to estimate a first LOS path to the RF tag and a second antenna orgroup of antennas is used to estimate a second LOS path to the same RFtag. Triangulating among the two LOS paths then provides an estimate ofthe RF tag's location in 3D.

The approach described above takes advantage of digital steering of theantenna array and can be cost effective in practice. In addition, theapproach can be conveniently scaled up to multiple antenna arrays. Theseantenna arrays can be distributed in a given space (e.g., on the ceilingof a store or warehouse) to ensure at least two antenna arrays have aLOS path with an RFID tag in the space. This can be particularlyadvantageous in an indoor environment where there might be multipleobstructions. Examples of indoor applications of this RFID approachinclude retail stores, libraries, and warehouses, among others (see moredetails below).

Digital steering can also be used to locate other RF transceivers,including those found in smartphones, wearables, tablets, laptops, andother portable electronic devices with WiFi, Bluetooth, or similarantennas. As with the RFID tag location described briefly above and ingreater detail below, a transmitter transmits a trigger signal to adevice with a WiFi, Bluetooth, or other RF transceiver. In response tothis trigger, signal the device emits a response that is detected by twoor more receivers via LOS and/or NLOS paths. A processor coupled to thereceivers steers the receivers' receptivity patterns by digitallysteering the AOA for different combinations of receivers (e.g., pairwisecombinations of receiver) and looking for the strongest signal(s) as afunction of AOA.

2 Systems for Distinguishing LOS and NLOS Paths

FIG. 1A shows a system 100 for distinguishing LOS paths 11 from NLOSpaths 13 to a device or item with an RF transceiver, such as an RFID tag10, smartphone, wearable computing device, tablet, or laptop. The system100 includes an RFID reader (transmitter) 110 and two receivers 120 aand 120 b (collectively referred to as receivers 120, also referred toas receiver antennas 120). The reader 110 and the receivers 120 arecoupled to a processor 130. Two receivers 120 are shown in FIG. 1A forillustrative purposes. In practice, the system 100 can include more thantwo receivers 120. These receivers 120 can be disposed in aone-dimensional (1D) or two-dimensional (2D) array. In another example,the receivers 120 are dispersed randomly or irregularly in a givenspace.

The receivers 120 can form (part of) a phased antenna array. In thiscase, the distance d between the two receivers 120 a and 120 b issubstantially equal to or less than half of the carrier wavelength λ ofthe radio-frequency (RF) signal used to interrogate the RFID tag 10,i.e., d≤λ/2. The system 100 can be configured to operate at any one ofvarious carrier wavelengths (accordingly various carrier frequencies).

For example, the system 100 can use RF signals in the ultra-highfrequency (UHF) region of the electromagnetic spectrum (e.g., about 850MHz to about 960 MHz) or microwave signals (e.g., 2.45 GHz). Thecorresponding carrier wavelengths are about 31 cm to about 35 cm for UHFsignals and about 12.2 cm for microwave signals. In this case, thedistance d between the two receivers 120 a and 120 b can besubstantially equal to or less than 17.5 cm at UHF frequencies or lessthan 6.1 cm at microwave frequencies. In other applications, such asoutdoor applications, the system may operate at lower frequencies (e.g.,13.56 MHz, 125 kHz, etc.) with correspondingly longer wavelengths (e.g.,22 meters, 2400 meters, etc.). For locating WiFi or Bluetooth devices,the system may operate in the unlicensed Industrial, Scientific, andMedical (ISM) band at 2.0 GHz to 2.4 GHz or at any other suitable band(e.g., 5 GHz). Higher frequencies (shorter wavelengths) generallyprovide more precise location estimates than lower frequencies (longerwavelengths).

The two receivers 120 a and 120 b include antennas 122 a and 122 b(collectively referred to as receiver antennas 122), respectively, toreceive the RF signal(s). The receiver antennas 122 can be controlleddigitally to change the phase difference of the signals that theyreceive from the RFID tag 10. This digital control can allow convenientsteering of the two antennas 122 toward different angle of arrivals(AOAs).

In one example, the reader 110 and the receivers 120 can be disposedinto a single enclosure to form an integrated device. The processor 130can also be integrated into the device. In another example, the reader110, the receivers 120, and the processor 130, can be distributed atdifferent locations. For example, the receivers 120 can be placed atlocations that have a clear field of view of the space to be monitoredfor the tag 10 (e.g., on the ceiling of a room), while the processor 130is placed at locations with better human access (e.g., in a controlroom). The readers 120 can be connected to the processor 130 via one ormore wired connections or over a wireless link (e.g., a WiFi link).

In operation, the reader 110 emits an RF signal towards the RFID tag 10.In one example, the reader 110 transmits the RF signal throughout agiven space, such as a room. In another example, the reader 110 emitsthe RF signal with a smaller divergence and steers or sweeps the RFsignal across the space. In either case, if the RFID tag 10 is withinthe given space, the RFID tag 10 can emit a response signal asunderstood in the art of RFID tags.

Depending on the position of the RFID tag 10 and the receivers 120, theresponse signal may propagate directly from the RFID tag 10 to thereceivers 120 along the LOS path 11 without being reflected orscattered. The response signal may also propagate in other directions aswell. For example, the response signal may be reflected or scattered offa wall 12 (or any other obstructions distributed throughout the givenspace). In this case, the response signal arrives at the receivers 120along the one or more NLOS paths 13. As described above, this can causemultipath issues and compromise the accuracy and reliability of thesystem 100.

The system 100 shown in FIG. 1 can distinguish signals along the LOSpath 11 from signals along the NLOS paths 13 based on their respectiveangles of arrival (AOA). The distinction can be made by determining theangles of arrival that correspond to extrema (e.g., local maxima andminima) in the antennas' receptivity pattern. For example, the systemprocessor 130 can coherently sum the signals received by adjacentantennas 122 at each of several phase differences, each of whichcorresponds to a different AOA. The coherent sums that produce maximacorrespond to the AOAs from which the LOS signals and NLOS signalsarrive. Absent attenuation, the tallest and steepest maximum generallycorresponds to the LOS path 11, with other maxima corresponding to AOAsfor the NLOS path(s) 13.

FIG. 1B shows the receivers 120 used to estimate AOAs θ based on thephase difference in the signals received by the two antennas 122 a and122 b. The RF signals 125 a and 125 b arriving at the two antennas 122 aand 122 b, respectively, can be regarded as substantially parallel toeach other provided that the distance d is small enough compared to thedistance between the receivers 120 and the RFID tag 10 (d<λ/2). In thiscase, the signals 125 a and 125 b have the same AOA θ with respect to anantenna plane 15 defined by the two antennas 122 a and 122 b. Withoutbeing bound by any particular theory or mode of operation, the phasedifference Δφ between the two signals 125 a and 125 b, as detected bythe two antennas 122 a and 122 b, respectively, can be written as:

$\begin{matrix}{{\Delta\phi} = {\frac{2\pi \Delta}{\lambda} = {\frac{2\pi d}{\lambda}{\sin (\theta)}}}} & (1)\end{matrix}$

where Δ is the length difference between the two paths taken by the twosignals 125 a and 125 b. Once the phase difference is determined, theAOA θ can be calculated according to Equation (1).

Equation (1) also represents digital steering of the antennas 122 towarddifferent AOAs θ. In this case, the phase difference Δφ between the twoantennas 122 a and 122 b can be adjusted by, for example, applying adigital delay to one or both antennas 122 a and 122 b. This digitaldelay offsets the propagation delay Δ shown in FIG. 1B. Once the phasedifference Δφ changes, the AOA θ changes accordingly, which means thatthe antennas 122 a and 122 b are steered toward a different AOA θ toreceive the signals 125 a and 125 b.

The phase difference Δφ can be changed across a range such that thecorresponding AOA θ changes from 0 to π. At each AOA θ, a correspondingsignal amplitude can be recorded. The signal amplitude can be a coherentsum of the signals detected by the two antennas 122 a and 122 b. Uponcompletion of the scanning of AOAs θ, a plot can be generated to showthe signal amplitude as a function of AOAs θ and find out the LOS path11 (see, e.g., FIG. 1C and the description below).

In the system 100, the processor 130 can be used to control the scanningof the AOA θ by controlling the amount of delay applied on the antennas122. The step size of the scanning Δθ can be about π/1000 to about π/10(e.g., about π/1000, about π/500, about π/200, about π/100, about π/50,about π/20, or about π/10, including any values and sub ranges inbetween).

The processor 130 can also exploit estimated, known, or measuredsymmetry to reduce scanning and/or processing time. For instance, theprocessor 130 may select and digitally calculate the phase difference Δφto steer the antennas 122 at symmetric angles (e.g., ±45°) instead ofasymmetric angles (e.g., −45° and +44°). Because the angles aresymmetric, they produce anti-symmetric results (e.g., results with onlya sign difference) and can therefore be calculated in about half thetime as asymmetric angles.

In addition, the knowledge of the antenna pattern can also be used toreduce the number of angles that need to be computed for givenmeasurement precision. For example, the sensitivity of the antennas 122may change rapidly around certain angles. At or near these angles, thestep size of the scanning Δθ can be reduced to sample more AOAs andproduce acquire more precise results. In contrast, at angles where thesensitivity of the antennas 122 stays relatively constant, the step sizeof the scanning Δθ can be increased to sample less, thereby reducescanning time and processing time.

FIG. 1C shows a plot 150 of notional signal amplitudes A versus AOA θ,i.e., A(θ), for RFID signals received by a pair of antennas 122 likethose shown in FIG. 1A. The upper trace 151 represents the compositesignal formed by digitally incrementing the phase difference between thesignals received by the antennas. In this case, the composite signalincludes a first maximum 155 a close to −3π/8 and a second maximum 155 bclose to +π/4. The second maximum 155 b is relatively tall and narrow(steep), whereas the first maximum 155 a is relatively short and wide.In this case, the tall and narrow second maximum 155 b corresponds to asignal that arrives at the receivers along an LOS path (e.g., path 11),whereas the wide and short first maximum 155 a corresponds to a signalthat arrives at the receivers along an NLOS path (e.g., path 13).

The processor 130 can be further used to correct for the antennareceptivity pattern S(θ) from the signal amplitude A, thereby yieldingthe rays 152 shown along the horizontal axis. This correction cansimplify identification of the angles of arrival corresponding to LOSand NLOS paths between the antennas and the RFID tag(s). Without beingbound by any particular theory of mode of operation, this correction maybe done by calibrating the antenna design and dividing out thecalibrated gain pattern or by effectively de-convolving the calibrationpattern from the measured signal amplitude A_(measured) (curve 151)since the measured signal is essentially a convolution of the truesignal A_(true) amplitude convolved with the antenna receptivity patternS(θ):

A _(measured)(θ)=A _(true)(θ){circle around (×)}S(θ)   (2)

This de-convolution can be used to recover the true signal amplitude asa function of AOA θ.

The antenna receptivity pattern S(θ) can be measured using a referenceantenna with a known emission pattern (e.g., A_(true)). With thisreference antenna as an illumination source, A_(measured) can berecorded. Then the receptivity pattern S(θ) can be calculated accordingto Equation (2).

After de-convolution, or other correction, the amplitude curve 151 isconverted into two peaks 156 a and 156 b. The higher peak 156 bcorresponds to the LOS path and the smaller peak 156 a corresponds to anNLOS path between the antennas and the RFID tag. If desired, theprocessor may fit curves (e.g., Lorentzians or Gaussians) to the peaks156 in order to generate a more precise estimate of the AOAs for the LOSand NLOS paths.

3 Estimating the Location of an RFID Tag

Based on the AOAs of LOS paths, the processor 130 can estimate theposition of the RFID tag 10 using triangulation. Two or more groups ofantenna arrays can be used. For example, a first antenna array, such asthe two antennas 122, is used to identify a first LOS path between theRFID tag 10 and the first antenna array. A second antenna array (notshown) is used to identify a second LOS path between the RFID tag 10 andthe second antenna array. The location where the two LOS paths crosseach other (or where the error between them is minimized) is the likelylocation of the RFID tag 10 in the plane of the LOS paths to the firstand second antenna arrays.

If desired, the processor may estimate the distance between each antennaand the RFID tag based on the amplitude or the received signal strengthindication (RSSI) of each LOS signal or based on slope of the differencein phase over difference in frequency. With two or more distanceestimates, the processor can trilaterate the RFID tag's location inaddition to or instead of triangulating based on the AOAs. Thesedistance estimates may be used to estimate the RFID tag's location moreprecisely or uniquely without AOAs.

The slope of the difference in phase over difference in frequency refersto a technique regularly used in radar and radar-like systems where thephase of the received signal is directly compared to the phase of thetransmitted signal. For an item (tag in this case) that is a givendistance from the reader, this phase offset should vary in a predictableway by its carrier frequency. So capturing this relative phase offset,ϕ, at multiple carrier frequencies, f, allows estimation of the distancefrom the reader to be:

$d = {\frac{d\varphi}{df}c}$

where c is the speed of light.

4 Training and Operating an RFID Tag Location System

An RFID tag location system may undergo a training phase before becomingoperational. In this training phase, the RFID tag location systemestimates the locations of reference RFID tags or other transceivers atknown locations. The system calibrates itself by comparing the referenceRFID tags' estimated locations to their actual locations. Once trainingis complete, the system can locate unknown RFID tags, smartphones,and/or other devices. The system may repeat training periodically (e.g.,at night, on the weekends, etc.) or as desired (e.g., in response touser commands).

To see how an example system (e.g., the system of FIG. 1A) determinesLOS and NLOS paths and estimates, consider a reader that emits acontinuous-wave (cw) RF interrogation signal at a wavelength of A. In afirst (training) phase, the reader interrogates a set of tags whoselocations are known. These tags are called reference tags. Eachreference tag receives this interrogation signal and emits a signal inresponse that is received in turn by each of k=1 . . . K antennas, eachof which is located along a line segment of length D wherex_(k)=kD/(K−1) is the lateral position of the kth antenna. (Otherantenna arrangements are also possible). Each antenna in the arraydetects the tag's output and emits a complex baseband signal s_(k)representing the tag's output.

If there is no multipath, each antenna's expected spatial response for atag at an angle of arrival θ is:

$\begin{matrix}{{w\left( {k,\theta} \right)} = {\exp \left( {{- j}\frac{2\pi}{\lambda}x_{k}\cos \theta} \right)}} & (3)\end{matrix}$

(It is not necessary to account for gain because the system is not abeamforming system.) The power received across the entire antenna arrayin the θ direction can be computed as:

B(θ)=|Σ_(k=)0^(K−1) w(k, θ) s _(k)|²   (4)

B(θ) is also referred to as the multipath profile of an antenna arraysince it (i.e., B(θ)) takes into account incident power from signalsalong both the LOS and NLOS paths. The system measures the multipathprofile of the antenna array at each of several AOAs for all of thereference tags and for one or more readers. Once the processor hasdetermined the AOA for LOS paths between the antennas and the referencetags, the processor can compute the reference tags' locations usingtriangulation and/or trilateration as described above.

The above technique can be extended to a 2D (or even 3D) antenna arraytopology. For example, for a simple 2D array in a 2×2 uniformrectangular array, assuming isotropic elements on the xy-plane, thesteering vector is given by:

${a\left( {\theta,\varphi} \right)}{\exp \left\lbrack {j\frac{2\pi}{\lambda}{\cos (\theta)}\left( {{p_{x}{\cos (\varphi)}} + {p_{y}{\sin (\varphi)}}} \right)} \right\rbrack}$

where the position vectors, p_(x) and p_(y) are given by:

$p_{x} = {{{\frac{d}{2}\begin{bmatrix}{- 1} \\{- 1} \\1 \\1\end{bmatrix}}\mspace{14mu} {and}\mspace{14mu} p_{y}} = {\frac{d}{2}\begin{bmatrix}1 \\{- 1} \\1 \\{- 1}\end{bmatrix}}}$

with d denoting the element spacing between the array rows and columns.

The power received at each 3D angle (θ, ϕ), and thus the 3D multipathprofile, is calculated by:

$\begin{matrix}{{B\left( {\theta,\varphi} \right)} = {{{a^{H}\left( {\theta,\varphi} \right)} \cdot s}}^{2}} \\{= {{\sum\limits_{k = 0}^{K - 1}{{a^{*}\left( {k,\theta,\varphi} \right)} \cdot s_{k}}}}^{2}}\end{matrix}$

FIG. 1D shows measured 3D multipath signatures for RFID tags atdifferent locations and angles of arrival with respect to a commonreceiver (antenna). Each plot shows the RFID tag signal amplitude versusazimuth and elevation angles. The peaks represent LOS and NLOS pathsbetween the tags and the antenna, with the tallest, steepest peak ineach plot representing the LOS path. These multipath signatures can becompared with each other to determine the relative AOAs and locations ofthe RFID tags as described in greater detail below.

After the system has completed training (it has measured the multipathprofile for all of the desired angles of arrival), it enters a second(operational) phase performed in the environment (or a duplicateenvironment). In the operational phase, the system interrogatesnon-reference tags (i.e., tags whose locations are unknown) and computesthe multipath profile for each unknown tag/reader combination. Thesystem compares the multipath profile for each unknown tag to themultipath profile(s) for the reference tags to determine the unknowntag's location.

The system may estimate the unknown tag's location by taking a weightedsum of three or more reference tag locations where the weights depend ondistances between the corresponding multipath profiles. For instance,the location of a reference tag whose multipath profile more closelymatches the multipath profile of the unknown tag may be weighted moreheavily than the location of another reference tag. The exact weightingcan be determined using a suitable distance metric, such as Euclideandistance, or “metric learning,” which leverages locations of bothreference tags and estimated locations of other unknown tags.Alternatively or in addition, the system may cluster reference tags andunknown tags according to a property (e.g., multipath profile) anddefine a representative example of that property for use in weighting.

The system may repeat the training phase, e.g., periodically, to accountfor changes in environment, such as changes in the number and locationsof the reference tags and changes in the number, type, and locations ofobstructions responsible for multipath effects. The system may also betested in a third (post-training) phase, in which an unknown tag ismoved through a series of known locations within the environment, e.g.,using a drone or robot. As in the operational phase, the system measuresthe unknown tag's location and compares that measured location to therobot or drone's coordinates for determining the optimal distance metric(metric learning) for weighting the reference tag locations.

In some cases, instead of calculating the solution (e.g., AOA and LOSpath) from one antenna array and then overlaying with another solutionfrom another antenna array, the raw data from both arrays can beacquired to generate a single composite solution. This can yieldmultiple solutions for arrays spaced farther than λ/2. Aliased solutionscan be ruled out by checking the plausibility of the resulting locationestimates.

5 Transmitters and Receivers for LOS and NLOS Determination

FIG. 2 shows an RFID system 200 including multiple readers 210 a-210 n(collectively, RFID readers 210) and multiple receivers 220 a-220 n(collectively, receivers 220). The RFID system 200 also includes aprocessor 230, a common local oscillator (LO) 240, and analog front ends250 a-250 n (collectively, front ends 250). Each reader 210 is groupedwith a corresponding receiver 220 and a corresponding front end 250, asshown in FIG. 2. Other arrangements of the readers 210, the receivers220, and the front ends 250 are also possible. For example, more thanone receiver 220 and/or more than one front end 250 can share a commonreader 210.

Each reader 210 includes a corresponding digital-to-analog converter(DAC) 218. The input of the DAC 218 is coupled to the processor 230 andthe output of the DAC 218 is coupled to a low-pass filter 216. Inoperation, the DAC 218 generates an analog representation of a digitalRFID tag interrogation signal generated by the processor 230. The filter216 removes high-frequency spurs and noise from the analog RFID taginterrogation signal. The output of the filter 216 is coupled to theintermediate-frequency (IF) input of a mixer 214. The LO input of themixer 214 is coupled to the LO 240. The mixer 214 mixes the analog RFIDtag interrogation signal with a high-frequency (e.g., 902-928 MHz)carrier from the LO 240 to produce an RF output, which is coupled to apower amplifier 212. The power amplifier 212 amplifies the RF output andcouples it to a circulator 256, which transmits the amplified RF outputto an antenna 252 a via a bandpass filter 254 a. The antenna 225 a canbe any suitable single antenna element. The circulator 256 substantiallyprevents the amplified RF output from propagating to or through thereceiver 220. The antenna 252 transmits the amplified RF output to theRFID tag, which responds with an analog response signal of its own.

The antenna 252 receives response signals from the RFID tag and couplesthem to the bandpass filter 254, which filters the response signal andcouples it the circulator 256. The circulator 256, in turn, couples allor substantially all of the response signal to a low-noise amplifier(LNA) 222. The LNA 222 boosts the amplitude of the response signal andcouples it to a mixer 224, which mixes the response signal with the LOto produce a down-converted RFID signal. A low-pass filter 226 removeshigh-frequency noise and spurs from the down-converted RFID signal,which is digitized by an analog-to-digital converter (ADC) 228 and fedto the processor 230.

The antennas 252 shown in FIG. 2 form an antenna array with a fixed orknown phase difference between pairs of adjacent antennas 252. Thecomponents and connections between components in the receivers 220 andfront ends 250 may be calibrated, tuned, lengthened, or trimmed toprovide known and stable phase relationships between the signalsreceived by nearest neighbor antennas 252. For example, at least oneantenna in each pair of adjacent antennas may be coupled to a phasetuner (not shown) to set or adjust the phase relationship between theadjacent antennas 252. The relative phase relationship between adjacentantennas 252 can also be measured and calibrated digitally using theprocessor (e.g., the processor 230 or a different processor not shown inFIG. 2). Maintaining fixed phase relationships between adjacent antennas252 allows digital steering of the antennas' receptivity pattern bydigitally adjusting the phase difference between the signals.

The system architecture shown in FIG. 2 can be used for locating anywireless system, including Bluetooth and WiFi; it would just operate atdifferent frequencies. A system that locates RFID, Bluetooth, and/orWiFi devices may include multiple copies of the components shown in FIG.2, with each copy for each type of device and operating in a differentfrequency band (e.g., 865-868 MHz or 902-928 MHz for RFID, 2400-2835.2MHz for Bluetooth, and 2.4 GHz or 5 GHz for WiFi).

6 Methods of Estimating RFID Tag Locations

FIG. 3A illustrates a method 300 for estimating the location of an RFIDtag, smartphone, or other device with an RF transceiver using a systemlike those shown in FIGS. 1A and 2. At step 302, a transmitter emits ortransmits an RFID tag interrogation signal to one or more RFID tagswithin a volume of interest, such as a store, stockroom, warehouse, orother environment where RFID tags are used. (Step 302 may be omittedwhen locating a device with an active transmitter, such as a cellular,WiFi, or Bluetooth transmitter.) The RFID tags respond to the RFID taginterrogation signal by emitting analog RFID signals, referred to asresponse signals. Two or more antennas receive the response signals atstep 304. One or more ADCs digitize the analog response signals at step306. In addition, electronic components coupled to the antennas can alsodown-convert and filter the analog response signals to facilitatesubsequent processing. The resulting digital RFID signals can be storedand processed in real time, post-processed, or both.

As described above, a processor coupled to the electronic componentsuses the digital RFID signals to identify the signals' AOAs with respectto the antennas. For example, the processor can electronically steer theantennas' receptivity pattern across one or more AOAs at step 308. Inone example, the AOAs can be selected a priori. For example, a uniformstep size (e.g., about π/1000 to about π/10) can be used to scan anglesbetween 0 and π. Alternatively or in addition, the AOAs can be selectedbased on previous measurements to reduce processing time. For example,at angles where the sensitivities of the antennas change rapidly, theprocessor can use a smaller step size to take more samples. In addition,the processor can use information about the RFID tags and theenvironment (including symmetry considerations) to select AOAs that aremore likely to yield a result in order to reduce processing time.

The processor can select likely AOAs based on principal componentanalysis (PCA) of previously received signals. For example, the antennascan monitor the movement of a specified RFID tag. Between successiveacquisitions of the response signals by the antennas, the RFID tag maymove by only a small amount ΔL, which can be much less than the distancebetween the RFID tag and the antennas. In this case, the AOAscorresponding to stronger signals in these adjacent measurements may besubstantially the same, so the AOAs estimated in previous measurementcan be used in subsequent measurement.

Each candidate AOA corresponds to a particular phase offset (alsoreferred to as the phase setting) as measured by the antennas. Thus, theprocessor can determine the signal strength for each AOA by digitallyadjusting the phase difference between the digitized RIFD signals fromtwo or more antennas with a known phase relationship (e.g., nearestneighbor antennas), then coherently summing the digitized RFID signalsat step 310. This steers the antennas' receptivity pattern through eachof the corresponding AOAs. It also yields the signal amplitudes andphases detected by the antennas as a function of AOA (phase differencebetween antennas). The steering generates a plot of the signal amplitudeas a function of AOAs (e.g., see FIG. 1C).

At an optional step 312, the processor can de-convolve, or otherwisecorrect, the antenna pattern from the plot of the signal amplitude (see,e.g., peaks 152 in FIG. 1C). This facilitates the determination of LOSpaths by examining the height of the peak. In general, the highest peakcorresponds to the signal that travels along the LOS path. For a moreprecise estimate, the processor may fit a curve to the peak, e.g., usinga polynomial or nonlinear regression, and estimate the AOA based on thecoefficients used to reduce or minimize the error associated with thecurve fit.

After the antenna pattern is de-convolved, or otherwise corrected, fromthe signal amplitude and phase, the processor may look for minima(valleys) instead of maxima (peaks). In this case, the processor mayidentify the LOS and NLOS paths based on the valley depths, valleywidths, valley slopes, or some combination thereof. For example, theprocessor may identify the deepest, steepest valley in a representationof the signal amplitude versus AOA as corresponding to the AOA for thenull along the LOS path to the RFID tag. Other valleys may correspond toangles for nulls of other NLOS paths to the RFID tag.

At step 314, the processor compares the amplitudes and phases at the AOAoffsets to determine the angle of arrival that is most likely torepresent the LOS channel for the RFID tag. The processor may identifythe LOS and NLOS paths based on the heights of the maxima, the widths ofthe maxima, the slope (rate of change) of the signal amplitude versusAOA, curve fit coefficients, or a combination thereof. For example, theprocessor can look for the tallest, steepest maximum in a representationof the signal amplitude versus AOAs. This maximum represents the angleat which a peak in the antennas' receptivity pattern is pointed alongthe LOS path at the RFID tag. Other maxima may represent angles at whichthe peak is pointed along NLOS paths at the RFID tag.

In some cases, the processor correlates the response signal with anexpected response from the RFID tag. This can be done, for example, atstep 308 in the method 300. In this case, the system (e.g., of FIG. 1A)may construct or use a library of expected responses from the RFID tag.Each expected response corresponds to a distinct AOA. The processorcompares the detected response signal with the expected responses tofind the closest expected response. The AOA of the closest expectedresponse is taken to be the AOA of the response signal. This techniqueis similar to matched filtering and can increase the signal-to-noiseratio (SNR) by up to 20 dB or more.

In optional step 316, the processor uses the different angles of arrivalfrom different pairs of antennas to estimate the RFID tag's location inthe environment. For example, the processor may triangulate the RFIDtag's location in at least two dimensions (e.g., in a plane parallel tothe floor) using two or more estimated angles of arrival in the sameplane. If the antennas are located in different planes, the processorcan estimate the RFID tag's location in 3D space based on three or moreestimated angles of arrival in different planes.

If more than two antennas are used in step 304, and these antennas arenot all co-linear, each RFID receiver can find an angle to the tag in 3Dspace. With this, in optional step 316, the location of the tag could bedetermined without the constraint of the antenna arrays being ondifferent planes.

In another optional step 318, the processor may track changes in a tag'slocation over time. More specifically, it may map smoothly varyingchanges in the tag's location to a path in 2D or 3D space. To do this,the system measures the tag's location at many points in time, e.g., ata rate of once every second or every few seconds. It computes the tag'slocation at each point in time, then makes vector distancedeterminations between successive locations to determine the tag'svelocity. The processor may classify the tag's velocity by speed anddirection and determine, based on the speed and direction, the tag'slikely trajectory and who is (likely to be) carrying or moving the RFIDtag. For instance, if the tag is moving at a walking pace towards theexit, the system may determine that a customer is taking an item withthe tag to the checkout or store exit. Alternatively, if the tag ismoving quickly to or from a storeroom, the system may determine that anemployee is stocking or shelving the item with the tag.

The system may also use measurements at many points in time todistinguish LOS signals from NLOS signals. If the system detects an LOSsignal and one or more NLOS signals, each of which appears as a separate“ghost” tag, it may construct trajectories for each signal. Thetrajectory for the LOS signal should vary smoothly, whereas thetrajectories for the NLOS signals may change direction sharply as thetag moves relative to antennas and the obstructions that scatter orreflect the NLOS signals. More specifically, the processor may usetime-varying measurements of LOS and NLOS signals to generate theprincipal vectors of the tag's trajectory. The vectors that solve for asmooth trajectory are likely to be LOS and the vectors that solve for arough trajectory or an impossible trajectory (e.g., due to some given orpredefined maximum velocity of a human) are thrown out as multipath(NLOS) rays.

In a system with multiple pairs of antennas (i.e., three or moreantennas), the processor may perform steps 302, 304, 306, 308, 310, 312,and 314 for different combinations of readers and antenna pairs toderive additional LOS and NLOS path information for one or more of theRFID tags in the environment. With a single reader and three or moreantennas, for instance, the processor can compute the angle of arrivalfor the LOS path to each pair of adjacent antennas. If the midpoints ofthe line segments connecting the different pairs of antennas are atdifferent locations, each pair of antennas can have an LOS path to theRFID tag with a different angle of arrival.

The processor can also perform steps 302, 304, 306, 308, 310, 312, and314 for combinations of a single pair of antennas and multiple readers.For example, the readers can emit signals that are synchronized in timeand phase such that the readers interrogate the RFID tag in a staggeredor round-robin fashion. The processor uses information about the timeand phase of the interrogation signals and the positions of each readerrelative to the antenna pair to determine the angles of arrival for theLOS and NLOS paths.

For systems with multiple readers, the processor may solve for the angleof arrival of each principal component of the detected signal based onthe position of the reader that triggered the signal. The processor maydetermine that those readers with coincident positions/intersectingangles of arrival share the same LOS path to the RFID tag. The processormay use this information to determine that other rays are the result ofmultipath (i.e., NLOS) instead of LOS paths.

Another method is to map the tag's trajectory to the trajectory of ahuman that is within view of one or more cameras. The cameras can bedisposed to monitor the same volume where the antennas are used tomonitor RFID tags. This method can be used in conjunction with the oneabove to provide a single trajectory as opposed to multiple trajectorieswith some vertical or horizontal shifts. More specifically, thecamera(s) can be used to detect groups of moving pixels (e.g.,representing a person or an object tagged with an RFID tag). A processorcoupled to the camera(s) determines the groups' trajectories and assignsthe RFID tag location to the group with a matching trajectory. Theprocessor can also segment out the human body or perform poseestimation. For instance, the processor may assess the difference intrajectory of a bag swinging in a person's hand versus the trajectory ofthe person.

The reader(s), antennas, and processor may perform steps 302, 304, 306,308, 310, 312, and 314 repeatedly. In one example, the steps 302 to 314are performed at regular intervals. For example, the steps can have arepetition rate of about 0.1 Hz to about 100 Hz (e.g., about 0.1 Hz,about 0.2 Hz, about 0.5 Hz, about 1 Hz, about 2 Hz, about 5 Hz, about 10Hz, about 20 Hz, about 50 Hz, or about 100 Hz, including any values andsub ranges in between).

In another example, steps 302 to 314 can be performed at predeterminedtimes, in response to a command or triggering event, or both. Forexample, steps 302 to 314 can be performed periodically (e.g., everyhour, every day, during evening inventory, etc.). They can be performedin response to the arrival of a new shipment, a stocking or re-stockingevent, a user command, or detection of a possible theft. For instance, astore manager may trigger the process 300 when the store opens in themorning and closes in the evening. Or the processor may trigger theprocess 300 automatically, e.g., at predetermined times or in responseto data from other sensors, including video cameras that monitor thesame space as the RFID tag location system.

In yet another example, steps 302 to 314 can be repeated more or lessfrequently in response to measured changes in the number or positions ofthe RFID tags. For example, the processor may determine that a firstRFID tag is moving if its position or corresponding LOS path angle(s) ofarrival change smoothly as a function of time. The processor maycorrelate the first RFID tag's movement with the movement of a personfrom video or image data of the person or information about a secondRFID tag, smartphone, or other RF transceiver carried by or affixed tothe person. If the processor correlates the first RFID tag's movementwith the person's movement, the processor may determine that the personis carrying the first RFID tag as well.

The processor may use this information about the first RFID tag'smovement, along with knowledge of the first RFID tag's position, totrigger other actions. For example, if the first RFID tag reaches aparticular area or volume or crosses a boundary surrounding an area orvolume, the processor may debit the person's account for purchase of anitem associated with the first RFID tag. The processor may also whenupdate a product inventory to reflect movement or purchase of theproduct or sound an alarm if the first RFID tag's movement isunauthorized.

7 Virtual Reference Tags

The systems shown in FIGS. 1 and 2 and process shown in FIG. 3A can beused to identify “virtual reference RFID tags” or “virtual referencetags,” which are RFID tags that can be used to generate precise locationestimates for other RFID tags. Using virtual reference tags providesgreater location precision for scenarios where there are multiple tagsbetween references: the denser the environment, the more precise thelocation. Virtual reference tags also enable measurements of relativedistances between items even if there are no non-virtual reference tags.For instance, it can be very useful to know that tag B is between tag Aand tag C even if the exact locations of tags A and C are unknown.

A simple way to view reference tags is shown in FIG. 3B, which shows asystem with several RFID readers 320 a-320 c (collectively, RFID readers320) that interrogate RFID tags in a store or other environment. TheRFID tag readers measure the RFID tags' LOS and NLOS signatures atdifferent angles of arrival (AOAs). The plots by each reader shows theLOS signatures for subsets of the tags. A processor 328 wirelesslycoupled to the RFID readers 320 compares these signatures to each otherto yield information about the relative positions of the RFID tags. Thisprocessor 328 may also be coupled to a remote server or computernetwork, such as the internet, for sharing and using information aboutthe tags' locations via a smartphone, tablet, or computer as describedin greater detail below.

The processor 328 may determine RFID tag position by fitting thesignatures to curves representing the RFID readers' receptivitypatterns, determining the peak (maximum) of each curve, andinterpolating between adjacent peaks to determine a Euclidean distancebetween peaks. This Euclidean distance represents an error or deviationin AOA for the corresponding RFID tags and RFID readers. For a pair ofAOAs, if neither is known, the error represents a difference in AOA(i.e., relative AOA); if one AOA is known, the other can be estimated.Multiple relative (or absolute) AOAs for a single RFID tag can be usedto estimate the RFID tag's relative (or absolute) position. Collectingmore data about the RFID tags, e.g., by making more measurements withmore RFID readers over more AOAs improves the location estimationprecision, e.g., to better than 50 cm, 40 cm, 30 cm, 25 cm, 20 cm, 15cm, 10 cm, or 5 cm.

FIG. 3B illustrates how this can be used to locate an RFID tag 322 at anunknown location with respect to other RFID tags and with respect to oneor more known positions. It shows a 1D view with reference tags 324 aand 324 b (collectively, reference tags 324) at known locations oneither end of a linear rack. Virtual reference tags 326 a-326 c andunknown RFID tag 322 are on the rack between the reference tags 324.

The plots beneath the first and second RFID tag readers 320 a, 320 bshow the RFID tag signal amplitude versus angle of arrival for thedifferent RFID tags. These profiles represent multipath signatures likethose described above, with the highest peak representing the LOS paths323 between the tags and the RFID tag readers 320. (The symbol aboveeach peak matches the symbol for the corresponding tag on the rack.)

There are two plots for the first RFID tag reader 320 a: the upper plotshows the multipath signatures without any obstructions between the RFIDtag reader 320 a and the tags, and the lower plot shows the multipathsignatures with a person 321 between the RFID tag reader 320 a and theleft-hand tags. Note that the person 321 attenuates/changes themultipath signatures for some tags, but not for others, and does notaffect the multipath signatures received by the second and third RFIDtag readers 320 b, 320 c.

The processor 328 may determine the relative positions of tags bycomparing their multipath signatures to each other. In this example, thetag signature for the RFID tag that is closest to either reference tag324 (e.g., RFID tags 326 a and 326 b) has the lowest error when comparedwith the tag signature of the corresponding reference tag 324. The errormetric used for comparing multipath signatures may be mean-squared error(MSE), dynamic time warping (DTW), or any other metric that can be usedto compare the similarity of the signatures. Using the example of MSE,the lower the metric, the more similar the multipath signatures are. Ina scenario where the multipath signature for RFID tag 322 is compared tothe multipath signatures for reference tags 324 a and 324 b, if theerror between the multipath signatures for RFID tag 322 and referencetag 324 a is smaller than the error between the multipath signatures forRFID tag 322 and reference tag 324 b, the processor 328 determines thatRFID tag 322 is closer to reference tag 324 a than it is to referencetag 324 b. If the error between the multipath signatures for RFID tag322 and reference tag 324 b is twice the error between the multipathsignatures for RFID tag 322 and reference tag 324 a, then RFID tag 322may be twice as far from reference tag 324 b as it is from reference tag324 a. Other non-linear weightings may also work.

If repeated RFID signal measurements show that the RFID tags 326 a and326 b aren't moving, they can be added as “virtual reference tags” evenif their absolute locations are unknown (at least to the same level ofprecision of the locations of the reference tags 324). This process canbe continued for other stationary RFID tags. For instance, RFID tag 326c is closest to RFID tag 326 a, so its RFID signature should be mostsimilar to that of RFID tag 326 a. If repeated measurements show thatRFID tag 326 c is stationary too, then it may be designated as a virtualreference tag as well. Continuing this process along, the system canestablish the order of the RFID tags (and hence the items tagged withthe RFID tags). The error metrics can be used as a proxy for relativedistance, and an estimate of the absolute location can be establishedbased on relative distances and the known locations of the referencetags 324.

Because the approach described above relies on the relative errors oftag signatures, it can be further improved by using signatures atmultiple readers, calculating the error by reader, and summing (orotherwise combining) the errors at the different readers. This is wherevirtual reference tags can be used to reduce location measurement error.Once the system has identified all of the stationary RFID tags,designated them as virtual reference tags, and located them with respectto at least their nearest neighbors, the processor 328 can locate thedesired RFID tag 322 with respect to one or more nearby virtualreference tags 326 based on their multipath signatures. By measuringerrors between different combinations of multipath signatures for theRFID tag 322 and the virtual reference tags 326, the processor 328 canimprove the precision of its estimate for the RFID tag's actuallocation, e.g., to within 50 cm, 40 cm, 30 cm, 25 cm, 20 cm, 15 cm, 10cm, or 5 cm of its actual location. The precision gets better as thenumber of virtual reference tags 326 goes up and the accuracy of eachvirtual reference tag's location improves.

The 1D example laid out above can also be extended to 2D by laying outthe reference tags in a 2D space (e.g., a wall or a floor) and comparingthe errors of tags, reference tags, and virtual reference tags withinthis space. This example can further be extended to 3D by laying thereference tags out in 3D space and comparing nearest neighbors,

This approach may be further improved by anything that changes the RFcommunications channel between the tag and the reader. This may includemoving the reader, changing the frequency the reader is operating on, oreven when a person (or object) moves in the space occupied by the readerand/or tag. For instance, consider a person 321 who walks between thevirtual reference tags 326 and the first and second RFID readers 320 a,320 b as shown in FIG. 3B. The person attenuates or scatters the RFIDsignals from the virtual reference tags 326 (and the unknown RFID tag322 and reference tags 324) propagating toward the first and second RFIDreaders 320 a, 320 b. This creates a new set of signatures at the firstand second RFID readers 320 a, 320 b that are not perfectly correlatedwith the set of signatures before the change in the channel, and canthus be used to reduce the error in the estimated locations of thevirtual reference tags 326 and unknown RFID tag 322.

Problems with using virtual reference tags tend to be around the largeamount of processing power used to compare every tag signature to everyother tag signature. The amount of processing power can be reduced byfirst comparing an RFID tag's signature to its last signature. If thesignature hasn't changed, there's no need to run a comparison. Otherways to reduce processing power include using sources of sideinformation (e.g., video of the region around the RFID tag and priorlocation information) to limit comparisons to signatures of the RFIDtags that are known to be close enough to each other to matter.

To identify or designate an RFID tag as a virtual reference tag, thesystem measures the RFID tag's location (e.g., using methods asdescribed above) several times over a period of time as the environmentchanges around the RFID tag. These location estimates may be distributedover an area or volume whose size depends on noise and the measurementuncertainty. As the number of measurements increases, the averagelocation estimate can converge to a smaller area or volume whose size islimited by the fundamental measurement uncertainty. Once the size of thearea or volume reaches a predetermined threshold, the processor sets anappropriate tag location (e.g., the centroid of the area or volume) anduses this tag location as a reference for similar tags. The system mayrepeat this process until a desired number or set of RFID tags have beenadded to the pool of virtual reference tags. Once a reference tag is set(the location is known), the method to calculate this reference taglocation is proven reliable and this method can be used to estimate thelocations of other tags.

The system may remove an RFID tag from the virtual reference tag pool ifthe RFID tag's location changes, unless similar RFID tags exhibitsimilar changes (e.g., due to environmental changes, such as obstructionby a person as shown in FIG. 3B). The system may identify changes in acohort of RFID tags by looking at the signature of each RFID tag in thecohort with respect to the signatures of its cohort members. The systemmay also look for changes (or a lack of change) in signatures receivedat other AOAs from the cohort. In FIG. 3B, for instance, the person 321may change the LOS signatures received by the first and second RFIDreaders 320 a, 320 b in a correlated fashion, but shouldn't affect theLOS signatures received by the third RFID reader 320 c. The combinationof correlated changes from certain AOAs and no change in other AOAsamong a cohort of RFID tags may indicate that the cohort isn't moving.

The change of the RFID location may be discounted in one or more of thefollowing situations. For instance, if there aren't any relative changesamong different RFID tags, it may be likely that all of the tags areobstructed or moving together. In another example, the relative changeis below a predetermined threshold (e.g., the measurement uncertainty).In yet another example, the relative change is brief enough (e.g.,within one, two, or three measurement cycles).

Information about virtual reference tags can be combined with otherinformation to increase RFID tag location precision. For instance, anRFID tag location system may use the (estimated) locations of virtualreference tags, product count data, and visual data from one or morecameras to determine the average density of products between virtualand/or real reference tags. Additionally, visual data can be used todetermine if a person is or was close enough to pick up or put down aproduct or RFID tag (pose/reach estimation can be done as well). If thevisual data shows that a virtual reference tag or product with thevirtual reference moves or has moved, the system may remove that virtualreference tag from the pool of virtual reference tags. Conversely, ifthe visual data shows that a particular RFID tag doesn't move for a longperiod (e.g., hours or days), then the system may designate that RFIDtag as a virtual reference tag.

8 Computer Vision System and Computer Vision System Training

The RFID techniques described above can be used to train a computervision system to locate and/or identify different objects captured bycameras in or coupled to the computer vision system. For instance, thecomputer vision system may include or be coupled to multiple cameras,which can be disposed to monitor a wide-angle area. In addition, lightsources emitting light at different wavelengths and/or intensities canalso be used to generate different environments so as to enhance thetraining of the computer vision system. Training images are acquired bythe cameras under different environments.

A combination computer vision/RFID tag location system cancross-reference timestamp data of scanned barcodes/transactions, theitems in the barcode/transaction, and camera data corresponding to thelocation of said register or checkout kiosk in order to pull frames thatcontain those items. A processor running object detection on thoseframes can draw bounding boxes around the imagery to produce additionaltagged imagery.

During the training of the computer vision system, such as a processorexecuting an artificial neural network, the RFID techniques describedabove are employed to locate and identify objects in the trainingimages. These objects are separated into discrete images, which are fedinto training sets for the computer vision system (e.g., an artificialneural network). If the computer vision system does not distinguishobjects that are too close together, reinforced learning can be used tofilter multiple objects. Additionally, the training can train thecomputer vision system to recognize objects, such as light fixtures,doors, and shopping carts, which may be irrelevant during the use of thecomputer vision system in, for example, a retail store. These objectscan be then removed from the training data set and frames containingimages of these objects may be marked as occluded frames. Since the RFIDtechniques described above can automatically identify objects with RFIDtags in an efficient manner, a massive database of product images can beconstructed without the need of humans to validate or verify thecontents of the database.

In certain cases, there may be error in the location of the tag relativeto the actual object. In these situations, one could plot the locationof tag from frame to frame relative to the locations of humans orobjects from frame to frame (e.g., attaining groups of moving contiguouspixels via optical flow or re-identification) and average the distancefrom each set of contiguous pixel groups to each RFID tag across nframes and group them based on which pairings resulted in the lowesterror/average distance. A Kalman filter would work well to filter/groupthe objects and/or objects. One could also incorporate the first andsecond derivatives of the RFID tag and pixel blob motion functions forweighting a match. If the goal is to attribute a product to a person,one can run human detection in each frame to only select pixels thatcorrespond to the human figure. If the goal is to gather annotatedimages, then one could use human detection to simply ignore the pixelsthat correspond to the human figure in the bounding box generation/pixelsegmentation.

FIGS. 3C-3F show a sequence of video frames showing movement of a shirttagged with an RFID tag. The small circle represents the estimatedlocation of the RFID tag which is attached to the shirt. The box isaround a pixel blob with which the system has correlated the RFID tagmotion derived from the RFID signals received by the RFID tag readers.As shown in FIGS. 3C-3F, there is significant error in where the systemestimates the RFID tag to be versus where the object actually is fromframe to frame, but the system is still able to attach the RFID locationestimate to a pixel blob.

FIG. 3G illustrates a process 340 for correlating RFID tag locationestimates with video data. This process 340 can be used to train neuralnetworks to recognize items tagged with RFID tags or to correlate motionof tagged and untagged objects (e.g., a garment with an RFID tag and aperson). The process 340 starts with recognizing, segmenting out, andignoring pixel blobs representing humans in the images with a trainedneural network (342). Then the remaining pixel blobs in the images areassigned a tracker which looks at their motion from frame to frame(344). Each pixel blob is then matched with the RFID tag that mostclosely follows its trajectory given a certain threshold (346). Oncepixel groups have been paired to respective RFID tags, one can runobject detection on each pixel group in each frame to throw out imagesthat clearly do not match the RFID tag description due to environmentalocclusions, such as bags, carts, jackets, humans, and other suchocclusion sources (348). Attributing tag data to a pixel blob alsoserves to reduce or eliminate the error between tag location and itemlocation.

On another note, the RFID location provides constant feedback to theartificial neural network such that it is always learning what it gotcorrect in each frame and what it got wrong. This extends to autonomouscheckout and human/product interactions where the process 340 can beused to teach the vision system on a frame by frame basis what was rightand what was wrong.

9 Tracking Moving RFID Tags

FIG. 4 illustrates how the RFID systems and processes described abovecan be used to track an RFID tag 402 moving in an environment filledwith obstructions, such as a store, stockroom, or warehouse. In thisexample, a pair of RFID tag readers 410 a and 410 b (collectively, RFIDtag readers 410) interrogate the RFID tag 402 by transmitting RFIDinterrogation signals at regular intervals, e.g., at a rate of about0.01 Hz to about 1.0 Hz. The RFID tag readers 410 may vary theinterrogation rate based on signals received from the RFID tag 410. Ifthe RFID tag's response signals indicate that the RFID tag is moving athigh speed, changing speed, or changing direction, the RFID tag readers410 may increase their interrogation rates to provide finerspatiotemporal resolution of the RFID tag's motion. Conversely, if theRFID tag's response signals indicate that the RFID tag is stationary ormoving slowly, then the RFID tag readers 410 may decrease theirinterrogation rates to conserve energy. The RFID tag readers 410 mayincrease or decrease their interrogation rates together or independentlydepending on the relative motion of the RFID tag 402.

The RFID tag readers 410 may broadcast the interrogation signals over awide range of angles, e.g., via isotropically emitting antennas, or scanthem over different angles with an antenna array as described above. Aprocessor (not shown) wirelessly coupled to the RFID tag readers 410uses the RFID signals from the RFID tag 402 to compute velocity vectors481 and a trajectory 491 for the RFID tag 402. To compute a givenvelocity vector 481, the processor may determine the RFID tag'slocations 471 at different moments in time, then determine a vectorconnecting those locations in 2D or 3D space. Scaling that vector by thetime difference yields the velocity vector.

The processor can use the location measurements and/or the velocityvectors 481 to determine the RFID tag's trajectory 491. This can be ahistorical trajectory (i.e., where the RFID tag 402 has been) or apredicted trajectory (i.e., where the RFID tag 402 is going based on itsestimated velocity). If desired, the RFID tag's current velocity, recenttrajectory, and/or predicted trajectory can be displayed on asmartphone, tablet, or other electronic device and used to trigger atransaction (e.g., a sale of an item associated with the RFID tag 402),prevent misplacement or theft, or track an item as it transits awarehouse as described in greater detail below. If the velocity vectors481 and trajectory 491 indicate that the RFID tag 402 isn't moving, thenthe processor may select the RFID tag 402 as a virtual reference RFIDtag as described above.

The processor can also use the location measurements, velocity vectors481, and trajectory 491 to distinguish a “real” RFID tag, such as RFIDtag 410 in FIG. 4, from an aliased or “ghost” RFID tag 482. In thiscase, the ghost RFID tag 482 is caused by multipath effects. Morespecifically, FIG. 4 shows that RFID signals propagating between thereal RFID tag 402 and the RFID tag readers 410 a, 410 b can take LOSpaths 411 a, 411 b, resulting in accurate measurements of the RFID tag'slocation, velocity, and trajectory. These RFID signals can also takeNLOS paths 413 between the real RFID tag 402 and the RFID tag readers410. In this example, some fraction of the RFID energy radiated by theRFID tag 402 reflects or scatters off a wall 412 to the first receiver410 a. And when the RFID tag 402 is in certain positions, this wall 412prevents it from sending RFID signals to the second receiver 410 b. Inshort, the wall 412 causes the first RFID tag reader 410 a to receivespurious RFID signals and stops the second RFID tag reader 410 b fromreceiving any RFID signals when the RFID tag 402 is between the wall 412and the first RFID tag reader 412 a.

In this case, processing the spurious RFID signals naively results inthe appearance of the ghost RFID tag 482 shown in FIG. 4, complete withghost locations 473, ghost velocity vectors 483, and a ghost trajectory493. The processor may distinguish the ghost tag 482 from acorresponding real tag 402 based on discontinuities in the ghostvelocity vectors 483 and ghost trajectory 493 and/or on similaritiesbetween the ghost velocity vectors 483 and real velocity vectors 481 andbetween the ghost trajectory 493 and the real trajectory 491. Inparticular, the real and ghost velocities and trajectories appear withmirror symmetry about a line or plane defined by the wall 412. Theprocessor can use this mirror symmetry and the abrupt discontinuities atthe beginning and end of the ghost trajectory 493 (where the wall 412begins and ends) to distinguish between the real RFID tag 402 and ghostRFID tag 482.

A camera 420 can be used to track the RFID tag 402 as well. In FIG. 4,the camera 420 takes pictures (e.g., at or around video rates) of aperson 401 carrying the RFID tag 402. The processor may use anartificial neural network to recognize the person 401 (e.g., as ageneric person, as an employee, or as a specific person) appearing inthe image and correlate that person's motion with the motion of the RFIDtag 402. If the RFID tag 402 is on a nametag, wristband, or ID card,this can be done as part of a process of training the neural network torecognize the person associated with the RFID tag 402. If the neuralnetwork has already been trained, then the processor may use theoverlapping or coincident motion of the person and the RFID tag 402 fortracking or to trigger another action, such as a sale of an item carriedby the person or to which the RFID tag 402 is affixed.

10 An RFID Tag Location System in Store With Retail Space and aStockroom

FIGS. 5A and 5B show different views of an RFID tag location system in astore 500. The RFID tag location system includes several RFID tagreaders 510 distributed throughout the store's sales floor 590,stockroom 592, and changing rooms 580. The RFID tag readers 510 can beplaced at or near the ceiling, e.g., as shown in FIG. 5B, to provideclearer lines of sight to RFID tags 502 on merchandise on the salesfloor 590 and in the stockroom 592. Placing the RFID tag readers 510above the RFID tags 502 also makes it possible measure each tag's 3Dlocation using azimuth and elevation information derived from the RFIDsignals received by the RFID tag readers.

The RFID tags 502 may be distributed throughout the store 500, includingon items for sale, such as clothing and other merchandise. The RFID tags502 may be embedded in the items or attached to the items with tags,clips, or stickers. There may be other types of RFID tags in the store500, including reference RFID tags 504 in known locations, such as fixedor movable clothing racks, walls, or tables. In addition, some of themoveable RFID tags 502 may be designated as virtual reference RFID tags506 if they remain motionless over a long enough period of time. Andsome RFID tags may be attached to ID cards 508 a and 508 b(collectively, ID cards 508), key fobs, bracelets, or other items wornor carried by employees 503 a-503 c (collectively, employees 503) orcustomers 501. These ID cards 508 may identify specific employees andtheir locations. Likewise, there may be RFID tags 502 embedded in orattached to shopping bags, baskets, or carts at an entrance 596 to thestore.

The RFID tag readers 510 communicate wirelessly with aprocessor/controller 530 via a wireless router 540 or another suitabledevice. Cameras, which are shown collocated with the RFID tag readers510, communicate with the processor 530 as well. The processor 530, inturn, can communicate with tablets 530 and smartphones 540 carried bythe customers 501 and employees 503. It may also communicate via asuitable communications network, such as the internet, with one or moreservers, databases, or other remote devices that track the store'sinventory and operations.

In operation, the RFID tag readers 510 measure the RFID tags' locations,velocities, and trajectories as described above. It uses thisinformation to monitor inventory and trigger actions associated with theitems tagged with the RFID tags 502. For instance, if the RFID tagreaders 510 detect an RFID tag 502 moving toward the store's exit 598and the cameras 520 detect a customer 501 moving along the sametrajectory, the processor 530 may trigger an automatic purchase of theassociated item by the customer 501. This enables the customer 501 toskip the checkout 582, saving time. The processor 530 may also directcustomers 501 and employees 503 to particular items based on preciseRFID location estimates by the RFID tag readers 520. This feature can beused to direct a customer 501 to a desired item, such as a shirt in aspecific size or color; to control and re-shelve inventory, such asitems left in the changing rooms; or to determine how inventoryplacement affects sales. These and other applications are described ingreater detail below.

11 Applications of High-Precision Object Location With RFID Technology

The RFID tag location techniques described above provide fine spatialresolution and high precision, making them suitable for a wide number ofapplications, many of which are not possible with other RFID taglocation techniques. Some of these applications are described below andcan be used with systems and in environments like those shown in FIGS.5A and 5B.

11.1 Tracking RFID Tag Movement

In one example, the RFID techniques can be used in retail stores, inparticular in omnichannel (also spelled omni-channel) stores.“Omnichannel” refers to a multichannel approach to sales that seeks toprovide customers with a seamless shopping experience, regardless ofwhether the customer is shopping online from a desktop or mobile device,by telephone, or in a brick-and-mortar store. What distinguishes theomnichannel customer experience from the multichannel customerexperience is that there is true integration between channels on theback end. For example, when a store has implemented an omnichannelapproach, the customer service representative in the store canimmediately reference the customer's previous purchases and preferencesjust as easily as a customer service representative on the phone or acustomer service webchat representative. Or the customer can use acomputer, tablet, or smartphone to check inventory by store on thecompany's website or app, purchase the item later via the website orapp, and pick up the product at the customer's chosen location.

One issue that retailers have with omnichannel orders is detecting thatan item has been picked by an omnichannel order. To address this issue,an RFID tag (referred to as a tote tag) can be placed on a tote, ashopping bag, a shopping cart, or any other suitable container. Eachitem for sale also includes or is affixed to a separate RFID tag(referred to as an item tag). Antennas arrays monitor the location ofeach tote tag and each item tag. If the distance between one item tagand one tote tag is below a threshold value (e.g., less than the size ofthe tote), the system determines that the item corresponding to the itemtag is in the tote. To improve the reliability of the detection, thesystem can further monitor the movement of the tote tag and the itemtag. If they move together for a distance above a threshold value (e.g.,more than 1 meter), the system can determine that the item and the toteare being carried by a customer.

The system can also monitor the movement customers to determine whetheran item is picked by the customer. For example, if an item movestogether with a customer for a distance above a threshold value (e.g.,more than 1 meter), the system can determine that the item is beingcarried by the customer. For example, a customer can install a User Appon his/her smartphone, and the system can detect the presence of thecustomer's smartphone via communication with the User App. The systemcan then track the movement of the smartphone (and accordingly thecustomer) using Bluetooth, WiFi, LTE, 3G, 4G, or any other wirelesstechnique.

In some cases, the system can maintain a record of all smartphones thatdo not belong to customers (e.g., store's own devices or employees'personal devices). Once the system detects a smartphone not in therecord, the system can determine that a customer enters the store andcan track the movement of the customer by tracking the smartphone.

The system may also track the movement of a customer using facialrecognition, gaiter recognition, or other recognition techniques. Forexample, a camera can be placed at the entrance of the store torecognize a customer, and one or more cameras can be distributed withinthe store to monitor the entire store space. Every time the customer iscaptured by a camera and recognized, the location of recognition can berecorded and compiled with previous locations to map out the movement ofthe customer. The resolution of this monitoring (e.g., distance betweentwo recognitions of the same customer) may depend on the number ofcameras in the store (e.g., a larger number of cameras can increase theresolution). The system can then determine that an item is picked by thecustomer if they move together for a distance greater than a thresholdvalue. Alternatively, or additionally, the system may determine that anitem is picked by the customer if they appear together at more than 3locations. The system may also determine that an item is picked by thecustomer if two locations where they appear together are more than 1meter apart.

The system can further update the inventory when it determines that anitem that was previously determined to be picked up by a customer isplaced back and available for sale. The system may determine that anitem is placed back if the item does not move for an extended period oftime (e.g., longer than 5 minutes). To improve the reliability, thesystem may also check whether there is any customer near the item whilethe item is not moving. In the absence of any customer staying near theitem, the system may determine that the item is placed back (e.g.,because the customer who previously picked the item changed his mind andabandoned the item).

In another example, the RFID techniques can be used to determine whetheran item is in the right place in the store. In this case, one or moretags (referred to as shelf tags) can be placed on each shelf that holdsthe items for sale. Each shelf tag identifies a particular location onthe shelf for a corresponding item. Each item also has an item tag. Forinstance, the shelf tag may indicate a location for men's pants, and theitem tag may be affixed to a pair of men's pants. The systeminterrogates the locations of the shelf tag and the item tag to estimatethe distance between them. If the distance is below a threshold value,the system can determine that the item is in the right place. If, on theother hand, the distance is greater than the threshold value, the systemcan alert one or more store employees that the item is in wrong placeand should be moved to the right place. The system may also provideinstructions to the employee(s) about the item's actual location and itsproper location.

The system can also determine whether an item is in the right placeusing tags attached to any other retail fixtures (these tags arereferred to as fixture tags). In general, each fixture tag can provideinformation about the identification of the fixture (e.g., shelf, table,counter, display case, basket, grid, etc.), the location of the fixture,and the type and quantity of items in the fixture. In some cases, thetype and quantity of items in a fixture can be determined based onindustry standards. Alternatively, the type and quantity of items in afixture can be customized for each store.

In addition, each employee can wear a tag (referred to as an employeetag). In one example, the employees can wear a bracelet including anRFID tag. In another example, the RFID tags can be sewn into theemployees' uniforms. In yet another example, the RFID tag can beincluded in badges worn by the employees. The system may use these tagsto estimate and track the employees' locations, e.g., for use inmanaging inventory as described below.

In some cases, the movement of employees can be monitored by softwarewithout using RFID tags attached to employees. For example, the systemcan monitor the movement of an employee by tracking the employee'ssmartphone. In these cases, the employee can install a User App tofacilitate communication between the smartphone and the system. Thesystem can recognize the employee from, for example, his or her useraccount on the User App.

In another example, the system can track the employee's wearable device,such as a smart watch, an activity tracker (e.g., Fitbit), or smartglasses (e.g., glasses with embedded electronics), among other devices.In this example, the system can maintain a record of wearable devicesthat belong to each employee so as to recognize the employee wheneverthe wearable device is detected. A system with a camera can also trackan employee via an RFID tag on an item that camera recognizes as beingheld or carried by the employee.

For instance, if the system determines that an item is misplaced orshould be brought from back stock to the right shelf, the system canestimate the locations of all employees using the employee tags. It maythen identify the employee who is closest to or moving towards themisplaced item. The system can alert that employee to place the iteminto the right place. The system can also estimate and/or measure thetime it takes the employee to complete the task (e.g., time durationfrom the alert to the completion). This information can be used toreview employee performance and identify changes to the store layoutthat could improve efficiency.

Several criteria can be used by the system to determine the appropriateemployee(s) to receive the alert. For example, the system can deliverthe alert to employees based on the employees' availability to receiveand respond to the alert. In this example, an employee can communicatehis availability (or unavailability) to the system via his employeedevice, such as a smartphone installed with a User App. The employee mayindicate that he is in the middle of other tasks that may not beinterrupted.

In another example, the system can send the alert to employees based ontheir proximity to item(s) at issue. For a misplaced item, the proximitycan be quantified by the distance between the employee and the misplaceditem. For an item to be placed, the proximity can be quantified by thedistance between the employee and the stockroom. In some cases, theproximity estimation takes into account the building or structure of thestore. For example, the system may preferably send the alert toemployees on the same floor as the item at issue, instead of sending thealert to employees on different floors.

In yet another example, the system can send the alert to employees basedon their ability to complete the task. For example, if an item in thewomen's clothing department is misplaced or an item is found abandonedin the women's fitting room, the system may preferably send the alert toan employee in the women's clothing department, instead of employees inthe grocery department.

The ability to complete the task may also be determined based on thecurrent tasks an employee is handling. For example, if an employee isalready handling some misplaced items, it may be more efficient for himto handle similar tasks. The system may also consult with the qualityassurance system to determine an employee's ability. For example, thesystem may include a database of employees' performance review of eachtask they handled before. If the system determines that an employeerestocks misplaced items with good efficiency, the system may preferablysend the alert to that employee.

In yet another example, the system may use a combination or weightedcombination of the above criteria to determine the most appropriateemployee to handle the issue. For example, the system may first find outemployees who are available. Then among these available employees, thesystem finds those who are within a certain distance to the item atissue. Out of these employees, the system can then determine the mostappropriate employee(s) based on the employee's ability to complete thetasks.

In some cases, the system may send the alert only to the mostappropriate employee (determined by any suitable method). Alternatively,the system may send the alert to a group of appropriate employees, andeach recipient can respond using his or her device (e.g., smartphone).Once a recipient responds by indicating that he or she is going tohandle the task, the system may update the status of the issue to, forexample, “in progress.”

The system may also send the alert to the supervisor(s) of appropriateemployees determined by the system. Alternatively, or additionally, thesystem may also copy the alert to personnel in quality assurancedepartment so as to alert them to monitor the progress of the issue.

RFID techniques can also be used to monitor the inventory availabilityof a store in a real-time manner. In this case, the system can track themovement of items that are picked up by customers. As described above,the system can determine that an item has been picked up by a customerif it is moving together with a tote. More specifically, the system mayuse the motion/trajectory of the item's RFID tag and themotion/trajectory of the tote as determined from video data and/or dataabout an RFID tag on or in the tote to determine that the item is in thetote. Once the system determines that an item is picked up, the systemdeducts that item from the available inventory. Alternatively, thesystem can deduct the item from the inventory until the item passesthrough the register where the item is checked out. In some cases, thesystem can also deduct an item, such as apparel or a pair of shoes, ifthe customer is wearing the item.

The RFID techniques can also facilitate validation of e-commerce orders,especially after the shipping box is sealed. RF signals can usuallypenetrate through shipping boxes so the RFID techniques described abovecan be used to identify items with RFID tags in the shipping box. Theidentified items are then compared with the order corresponding to thisshipment so as to determine whether any item is missing, or any itemshould not be in the shipping box. If a missing item is confirmed, thesystem can check the inventory or other database to find out whether areplacement item is in the distribution center (DC) or a nearby store.The system can also prevent the shipping box from leaving the storeand/or DC until the item is placed in the shipping box or located forseparate shipment.

In some cases, the RFID techniques can be used in dressing rooms totrack items that are tried on by customers. The system can determinewhether an item left in the dressing room has been in the dressing roomfor a longer than a threshold time (e.g., longer than 15 minutes).Alternatively, the system can track the location of the item as well asthe status of the dressing room. For example, in the case when thesystem determines that an item is in the dressing room and the dressingroom is unused, the system can determine that the item is left in thedressing room. In these cases, the system can alert an employee to pickup the item and place it back on a shelf for sale.

The system may determine the status of the fitting room by tracking thepresence of a customer's mobile or wearable device in the fitting room.For example, the system can generate a map of the fitting rooms anddisplay mobile and wearable devices detected in each fitting room. If nodevice is found in a fitting room, the system can indicate that thefitting room is probably unused. In this case, an employee can go to thefitting room to pick up abandoned items.

The system may also determine the status of the fitting room using anRFID tag attached to the door of a fitting room (e.g., on the movingedge of the door). In this case, the door of the fitting room can bedesigned to move away from the frame if it is unlocked. Therefore, theRFID tag is at a first location when the door is closed or locked (i.e.,when the fitting room is occupied) and at a second location when thedoor is open or unlocked (i.e., when the fitting room is unoccupied).The system can then determine the status of the fitting room based onthe location of the RFID tag on the door. Similarly, another option isto install several reference tags in or on the fitting room curtain anddetect them shifting closer together or farther apart as a result ofsomeone opening and closing curtain.

Alternatively, each fitting room can use two RFID tags: one is placed onthe moving edge of the door and the other is placed on the frame of thedoor. Alternatively, the RFID tags can be placed on or integrated intodifferent parts of a lock on the fitting room door. The system can thendetermine the distance between these two tags. If they are within athreshold value (e.g., about 10 cm), the system can determine that thedoor is closed or locked; otherwise, the system can determine the dooris open or unlocked and the fitting room is unoccupied.

Additionally, the system may use a combination of location and amplitudeshift in the RFID signal to determine if a garment with an RFID tag ison a person. For example, if the garment is suspended in mid-air in themiddle of the fitting room, it is likely on the body. If the RFID taglocation system detects a noticeable dip in RSSI accompanied by anindication (e.g., from camera data) that the RFID tag is close to theperson's body, it may determine that the object/garment tagged with theRFID tag is likely on the person's body.

11.2 Shelving Items With RFID Tags

Precise tracking of items also allows the system to place items onshelves using autonomous vehicles (e.g., robotic devices, drones, etc.)without human intervention. For example, an RFID tag can be attached toeach item providing information about the desired location of the itemin the store. The autonomous vehicle can include a tag reader to readthe RFID tag and deliver the item to the desired location. The desiredlocation (e.g., a designated shelf) can also be marked by an RFID tag(referred to as a fixture tag). In some cases, the autonomous vehicleuses its internal tag reader to locate the fixture tag and estimate thedistance and direction from its current location to the fixture tag anduse the estimate to navigate toward the fixture.

In some cases, the system can monitor the location of a remotelycontrolled vehicle using an RFID tag on or embedded in the vehicle. Ifdesired, the system or a user may direct the vehicle to move toward thefixture. In these cases, the vehicle may not include any tag reader.

Alternatively, the RFID tag can include the item's identificationinformation (e.g., a serial number), but not the item's desired orintended location. Instead, the identification information is associatedwith the desired location information in a database. An RFID tag reader(e.g., on an autonomous vehicle) can read the identification informationand communicate with the database to retrieve the location information.

Automatic shelving with autonomous vehicles can be performed every nightafter closing of the store and/or every morning before the opening ofthe store. In some cases, the shelving procedure is automatic such thatit can be performed without human monitoring. Accordingly, the shelvingcan be performed after hours so as to save overtime cost.

In some cases, the shelving can be performed on demand. For example,when the system determines that an item is on demand, the system cansend a person or robot to the stockroom to pick one item and deliver theitem to the desired location. In some cases, the person or robot mayalso be directed to pick a misplaced item and place it in the correctlocation. The system can direct the robot to the location of themisplaced item and to the item's desired location. In some cases theRFID tag data and/or camera data may also reveal orientation of theobject and other information, such as weight, geometry, and weightdistribution, to aid in complex problems such as grasping.

11.3 Monitoring Inventory of Items With RFID Tags

The system can monitor the inventory of items based on the precisetracking of the items' locations with RFID tags. As described above, thesystem can determine that an item has been picked or is being carried bya customer, in which case the system can remove this item from the listof available items. The system can further place this item to atemporary list of items under consideration for purchase by thecustomer. Once the item is checked out by the customer (e.g., in theevent that the customer leaves the store with the item), the system canremove this item from the temporary list. If, however, the customerchanges his mind and places the item back (or simply abandons the item)before checking out, the system can put this item back to the availablelist.

In some cases, once the system determines that an item is underconsideration by a customer, the system can interrogate the RFID tagattached to the item at a frequency greater than 1 Hz to track themovement of the item. Once the item is placed back to the shelf, theinterrogation frequency may be reduced to reduce computing burden of thesystem.

Employees may participate in the inventory monitoring by handlingdefective items. A defective item may be identified by an employee or acustomer. In either case, the employee may use an employee device toscan the RFID tag on the item and enter the status (e.g., “defective” or“damaged”) of the item to the system. The employee device can include atag reader and an interactive interface (e.g., a touch screen) for theemployee to update the inventory. In response to receiving the status,the system can remove the item from the available list and put the iteminto another list (e.g., repair list or return list). The system canalso send one or more alerts to relevant personnel to handle thedefective item.

11.4 Employee and Product Location Using RFID Tag Location System

The RFID tag location system may include a plurality of cameras, RFIDtags, and wireless communication systems such as Bluetooth or Wi-Fi totrack the precise locations of employees and products within a store.The location of employees and products may be derived from RFID taglocation data collected by the RFID tag location system and thendisplayed using a GUI on a smartphone or tablet. FIG. 6 shows an exampleGUI that displays the location of an employee and several products on afloor plan of the store. Since the RFID tag location system can identifythe exact locations of both employees and products, the relativelocations between the employees and products can also be displayed, asshown in FIG. 6. In addition to displaying the location of products on afloor plan, as shown in FIG. 6, the location of products may also bedisplayed in a virtual tour of the store, in a 3D view of the store, orin an online shopping feature.

11.5 Product Selection Using the GUI

The GUI, as described, can display one or multiple products on a floorplan of the store. The GUI may also allow a user, an employee or acustomer, to interact with the displayed products in order to performspecific actions. User interaction with the GUI may be accomplished byseveral methods including a pointing device, such as a mouse, atouch-based system, such as a user's fingers or a stylus, and so on. Forexample, in a touch-based system, the user may select one or moreproducts by drawing a shape around the products displayed in the GUIwith their finger. This process is illustrated in FIG. 7A through FIG.7D. FIG. 7A shows numerous products displayed in the GUI. The user mayuse their finger to begin drawing a circular shape around multipleproducts, as shown in FIG. 7B, until completing the circular shape asshown in FIG. 7C. The products contained within the circular shape arethus selected. Prior to a user specified action, information on theselected products may be displayed such as the number of productsselected, as shown in FIG. 7D.

Once products are selected by the user, the user may then specifynumerous actions to be performed on the selected products. These actionsmay include the following: (1) listing product details, e.g., stylenumber, color, price, size, etc., (2) listing quantity or value sold,(3) instructing the RFID Reader to only read the selected products,e.g., during receipt of a new shipment, conducting an inventory count,etc. (4) changing the floor display of specific products, (5) choosingto receive a price alert on selected products, (6) displayinginformation on similar products, (7) receiving recommendations onsimilar products or newer models of selected products, or (8) havingselected products delivered or picked for purchase. Some actions mayonly be available to either employees or customers depending on theirfunction.

11.6 Updating Inventory and Automatic Notifications of New Shipments ofProduct

The RFID tag location system may also be used to facilitate updates toinventory when new shipments of product arrive at store. For example, ashipment may be delivered to a store from a manufacturer, warehouse,distribution center, or another store. To verify the quantity of productin the shipment, an RFID reader and a User App can be used. The RFIDreader can be optimally located in a stores shipment process area suchas the stockroom, the sales floor, or other locations a retailer may useto process inbound shipments.

Products contained in the shipment may or may not include RFID tags. Forproducts that do have RFID tags, employees can use the RFID reader and aUser App to verify the quantity of products received matches acorresponding invoice for the product order. For products that do nothave RFID tags, employees can add RFID tags to the product and encodethe tag with appropriate product information using the RFID reader andUser App. These products can then be added prior to confirming thequantity of product received in the shipment.

Once the quantity of product received is verified, the RFID stock of theproduct and a master corporate stock of the product, which may includeproducts with and without RFID tags across multiple stores, will beupdated to show accurate stock levels of the product at the store wherethe shipment was received and at a corporate level across multiplestores. If a discrepancy were to exist between the quantity of productreceived and in the invoice, the RFID tag location system can facilitateresolving said discrepancies by checking whether products arrived in theshipment, products arrived without RFID tags, and products arrived withincorrect RFID tags.

Electronic notifications may also be automatically sent to customersnotifying them of delivery of new shipments of product to a store. Thenotifications may be sent using a variety of methods including e-mail,text message, messaging applications such as WhatsApp, Facebookmessenger, Geofencing applications, or other electronic message servicesintegrated with the RFID tag location system. The notifications may betailored to Priority Products, e.g., new products, best-sellingproducts, or products elected by customers for notifications, based oncustomer preferences. The notifications may also be sent to customerswho have visited a specific store previously or who have subscribed toreceive notifications from a specific retailer or retail location.Retargeted advertisements or electronic messages may also be sent tocustomers who previously visited a store and were unable to purchase aparticular product due to lack of availability, e.g., a Priority Productwas not available in a desired size.

The RFID tag location system can also facilitate discovery of productsmissing RFID tags or have incorrect RFID tags after delivery or duringinventory checks. For example, an employee may discover upon inspectinga pile of identical clothing that the stock level of the clothing iszero, which indicates an error in the RFID stock due to missing orerroneous RFID tags. In another example, an employee may be carrying aparticular product and visually notice the product is missing an RFIDtag. In the event products are discovered to have missing or erroneousRFID tags after receipt and verification of a shipment, employees canadd or replace an RFID tag to the product and encode the tag with thecorrect product information using the RFID reader and User App. Once anew RFID tag is encoded, the RFID stock of the product will be updatedand automatic electronic notifications may be sent to customers, asdescribed earlier.

11.7 Automatic Notifications for Product Movement and Holds

The RFID tag location system can also monitor product movement, e.g., ifa Priority Product has not been moved to an appropriate location such asthe sales floor, or product holds, e.g., a product is not placed on holdfor a customer. Based on the RFID tag location data, an electronicnotification can be sent to authorized employees, regional managementpersonnel, or corporate management personnel if product movement orproducts holds do not occur within a certain time threshold establishedby the retailer, e.g., 30 minutes. The notifications may be sent using avariety of methods including e-mail, text message, messagingapplications such as WhatsApp, Facebook messenger, Geofencingapplications, or other electronic message services integrated with theRFID tag location system.

11.8 Product Status Based on RFID Tags

The RFID tag location system may also encode additional information inthe RFID tag of a product. For example, an RFID status tag may be used,which can include a variety of product statuses and trackinginformation. The RFID status tag is distinguished from a RFID tag wherethe RFID status tag can assign the same product information to a set ofRFID tags that correspond to the RFID status tag.

Numerous product statuses may be encoded into an RFID tag and may bebased on categories including Transfers Out, E-Commerce Orders, andDamages. In the Transfers Out category, product statuses may include (1)merchandise that will be sent from a first store to a second store, awarehouse, or a distribution center, (2) time and date of product statuscreation, (3) type of Transfer Out, e.g., transfer to a different store,transfer to a warehouse, transfer to a distribution center, transfer ofdamaged or recalled products, transfer of products to be cleaned ortailored if such services occur off site from the store, (3) origin ofmerchandise, e.g., store number, (4) destination of merchandise, e.g.,store number, distribution center, manufacturing facility number, or (5)a transfer out number, e.g., a tracking number created by the RFID taglocation system or existing legacy systems.

In the E-Commerce Orders category, product statuses may include (1)merchandise that will be sent from a store to a third party shippingaddress specified by the customer placing the order, (2) time and dateof product status creation, (3) origin of merchandise, e.g., storenumber, (4) customer account number, e.g., account created by thee-commerce system, (5) an e-commerce order number, e.g., an order numbercreated by the RFID tag location system or existing legacy e-commercesystems, or (6) e-commerce status, e.g., IN-PROCESS for products pickedup and currently in processing area waiting to be packed or PACKED forproducts that have been picked and packed for outbound shipment.

In the Damages category, product statuses may include (1) merchandisecurrently unavailable for sale due to contamination, damage, or defects,(2) time and date of product status creation, or (3) a damage transfernumber, e.g., a reference number created by the RFID tag location systemor existing legacy systems.

The use of RFID status tags can facilitate assignment of a productstatus in a particular area of a store based on the location accuracy ofthe RFID tag location system or by product type. For example, an RFIDstatus tag on a particular product can automatically assign the samestatus to other products in its immediate proximity, e.g., products thatare within 4 inches of the product with an RFID tag. In another example,the RFID status tag on a particular product can assign the same statusto a group of products across an entire store. A change in status for agroup of products can be displayed in the GUI using a different color orsymbol for these products. This visual indicator can help an employee toverify the status for the products.

The RFID tag location system can also automatically change productstatus based on RFID stock of a product. For example, products with RFIDtags that have Transfer Out statuses, e.g., transfer to another store,transfer to a warehouse, or transfer to a distribution center, can beconsidered as available stock for e-commerce orders to be fulfilled bythe store sending said products so long as the transfer out process isnot confirmed by the store. A confirmation may include the product is ina sealed box, transfer documentation is finalized, etc.

11.9 Tracking Arrival and Departure Routes of Products

The RFID tag location system, which may include an RFID reader, (depth)cameras, and technology to accurately determine the location of RFIDtags, can be used to record the path of one or more RFID taggedproducts, e.g., products with RFID tags or RFID status tags grouped in abox, bag, or cart, through a store as the products enter or leave thestore. Using the User App, the path can then be displayed in the GUI toa user as an animation overlaid on a floor plan of the store as shown inFIG. 8.

The RFID tag location system can also play back a recorded video feed ofthe arrival or departure of the RFID tagged products using the date,time, and location data recorded by the system's location technology andcameras as shown in FIG. 8. Additionally, the RFID tag location systemmay also identify the individual accompanying the RFID tagged productsbased on facial or gait recognition, an individuals' Bluetooth or Wi-Fienabled device, or a user ID.

To ensure a store is fully covered by the RFID tag location system, thecomponents of the RFID tag location system may be mounted on ceilings orwalls every 500 to 1000 square feet increments depending on the layoutand environment of the store. This enables the RFID tag location systemto track all RFID tagged products and Bluetooth or Wi-Fi_33 enableddevices within the store. Additionally, the system can also identify theboundaries of the store, e.g., multiple floors, rooms, entrances, exits,etc. The boundaries of the store may be marked with RFID reference tagsor other manual marking methods for detection. In particular, byidentifying entrances and exits, the RFID tag location system canautomatically register when products have entered or exited the store.

11.10 Smart, Adaptive Floor Display of Product Quantity

The RFID tag location system can enable a user, e.g., an employee, toset a desired quantity of product to be located in a particular area ofthe store, e.g., 12 units of a product on the floor display.Furthermore, the RFID tag location system can suggest an ideal placementof products to the user based on historical data on the performance ofthe product in order to maximize sales. For example, a specific productmay have multiple variants, e.g., footwear, apparel, accessories,women's wedding dresses with different sizes. The RFID tag locationsystem can suggest to users the highest performing product variants toput on a floor display for that particular store. Historical performancedata can include historical sales, number of times a product or productvariant is viewed or tested by a customer, or conversion rates of aproduct or product variant, e.g., views to sales, customer tests tosales, etc.

The RFID tag location system can also dynamically adapt to the quantityand location of products in a store in real-time based on stockinventory available at the store. For example, in Table 1, an idealscenario is shown where the M and L sized products are the bestperforming variants followed by S and XL sized products. Based on auser-defined requirement on the total number of products to display,e.g., 12 in this example, the RFID tag location system automaticallycalculates the number of products for each size to put on the floordisplay. In this case, more M and L sized products are shown than S andXL sized products since they are higher performing.

TABLE 1 Ideal Scenario Total # to Display 12 S M L XL Floor DisplayQuantity 2 4 4 2 Sales Floor Quantity 3 1 3 2 Stock Room Quantity 10 128 9

In another example, Table 2 shows an adapted scenario where M sizedproducts are insufficiently stocked and thus cannot fulfill the idealfloor display previously shown in Table 1. In response, the RFID taglocation system reallocates the number of product variants to put on thefloor display based on the next best performing product variant. Thisdoesn't necessitate that zero M sized products are displayed, but ratherthe number of M sized products is reduced to accommodate available stockand customer demand. In this case, more L sized products are displayedfollowed by S and XL sized products.

TABLE 2 Adapted Scenario A Total # to Display 12 S M L XL Floor DisplayQuantity 3 1 5 3 Sales Floor Quantity 3 1 3 3 Stock Room Quantity 10 0 89

Table 3 shows yet another adapted scenario where both M and L sizedproducts are sold out and there is an insufficient stock of otherproduct variants to meet the required total number of products. In thiscase, the RFID tag location system will reallocate the number of productvariants to fulfill the total number of products to display as best aspossible while prioritizing best performing sizes as well.

TABLE 3 Adapted Scenario B Total # to Display 12 S M L XL Floor DisplayQuantity 6 0 0 3 Sales Floor Quantity 3 0 0 3 Stock Room Quantity 3 0 00

The RFID tag location system will also set the quantity of a product onthe floor display to zero if the sales floor quantity is set to zero.Furthermore, notifications may be sent to employees if at least oneproduct is on the sales floor, but no products are on the floor display.This is based on one possible retailer strategy where all productsavailable on the sales floor should also be placed on the floor display.The RFID tag location system may also be configured to detectdiscrepancies in the quantity of product on the sales floor and thefloor display, particularly to compensate for input errors to thesystem.

11.11 Creation and Optimization of Pick Lists

As described earlier, the RFID tag location system can accurately trackthe quantity of products located in different areas of a store, e.g.,the sales floor or the stockroom, and can thus determine what productsor product variants may need to be moved to the sales floor inreal-time. For example, Table 4 shows a distribution of product variantsin a store. As shown, there is an insufficient number of M and XL sizedproducts available on the sales floor based on the number of productsshown on the floor display. As a result, two M sized and one XL sizedproducts should be moved from the stockroom to the sales floor.

TABLE 4 Distribution of Products in a Store Total # to Display 12 S M LXL Floor Display Quantity 3 3 3 3 Sales Floor Quantity 3 1 3 2 StockRoom Quantity 10 12 8 9

To facilitate replenishment of a products or product variants, the RFIDtag location system can instantaneously compile a pick list, or a listof requested products, that need to be replenished in real-time. Thepick list can then be sent to a user, e.g., a stockroom employee, whothen completes the request by pick up all requested products anddelivers the products to the sales floor.

The use of a pick list may also be applicable to fulfill e-commerceorders where the RFID tag location system compiles a list of productsrequested by online customers to be picked up at the store. The productsin the pick list may also be placed on hold by an employee on behalf ofthe customers, by a customer using a retailer's website or application,or by a customer variant of the User App. Pick lists may also be used incustomer stock requests, where products are requested from the stockroomby an in-store customer via a sales floor employee, or for misplacedproducts, where products are placed in incorrect locations on the salesfloor or in the stockroom.

Since the RFID tag location system can track the location of a user,e.g., a stockroom employee, and the locations of all products in thepick list, an optimized pick path (OPP) can be generated, based on theshortest time or distance for an employee to pick up all products. TheOPP can be displayed to the user in the GUI of the User App. In FIG. 9A,the OPP is displayed as a dotted line along with the locations of thenearest product on the pick list and the user. The OPP will update asthe user moves, as shown in FIG. 9B. As the user begins picking upproducts on the pick list, the OPP will continue to update and will alsoshow the number of products picked up by the user as shown in FIG. 9Cand FIG. 9D. Also, the next product to be picked up by the user willalso be displayed in the GUI. An OPP may also be used for e-commerceorders, customer stock checks, and moving misplaced products in thestockroom or on the sales floor.

In the event a product on the pick list of a first user is picked up anddelivered to the sales floor by second user and the first user is stillin the process of fulfilling the request and before picking up saidproduct, the RFID tag location system will specially mark the product onthe first user's pick list to notify the first user that the product isno longer needed. This notification process can be performed inreal-time using the RFID tag location system.

11.12 Pick List Filters

The RFID tag location system can also enable a user to refine a picklist based on product attributes or location. For example, a user mayfilter a pick list according to women's wedding dresses, stockroom 1, orwomen's wedding dresses in stockroom 1. Furthermore, a user can set amaximum quantity of products to be included in a pick list, e.g., 10units. The RFID tag location system will then show a pick list with upto 10 units. Based on user filters and the maximum quantity, the RFIDtag location system can optimize the products on the pick list thatgenerate the most sales for a store.

11.13 Stray Products

Stray products are products that are placed in incorrect locationswithin the store, e.g., a product is designated to be on the salesfloor, but is instead located in the stockroom. The RFID tag locationsystem can actively and accurately track the location of units ofspecific products, e.g., all units of a men's black V-neck T-shirts areall located on the sales floor or in the stockroom. The combination ofthe RFID tag location system's location accuracy and ability to monitorall units of a particular product can enable automatic detection ofstray products in the store. If stray products are detected,notifications may be automatically sent immediately to users, e.g.,employees, or sent after a user-defined time threshold, e.g., greaterthan 10 minutes, that units of a product are in an incorrect location.

Furthermore, the User App can also generate a path within the GUI todirect a user to all stray products. This path generation feature canalso be used for non-stray products as well. By way of example, FIG. 10shows a GUI where a specific product is selected within a store. Theunits of the selected product may not be located in the same stock roomnor in the same area of a particular stockroom, e.g., the units of aproduct may not be located within six feet of one another. In theseinstances, the GUI may display to the user the total number of locationsthe user must visit to retrieve all units of a product.

11.14 Intelligent Routing of Product Notifications to Users

The RFID tag location system, in particular the RFID reader, the UserApp, and the location tracking features of the system, can be used tomonitor the location of a product, or a variant of the product,accurately. By tracking all RFID-tagged products within a store, thesystem can automatically notify users, e.g., employees, if a productneeds to be replenished in a particular area of the store in real time.The threshold or criteria for product replenishment can be by the users.For example, a product may be required to have 10 units located on thefloor display. If initially there are 10 units of a product on the floordisplay and a customer purchases 1 unit, a notification can be sent toan employee that the quantity of product on the floor display has fallenbelow the specified requirement, prompting the employee to transfer 1unit of product to the floor display.

An employee receiving a notification for product replenishment may alsosend a request for the product to another employee, e.g., a sales flooremployee can request a product from a stockroom employee, using anInternal Stock Request. In the event the requested product is not instock at the first store, an employee can instead use an External StoreRequest to request the product from a second store or warehouse anddelivered to the first store or to a customer's preferred address. Thistracking feature can also be used my customers using the User App withtheir mobile device to locate a particular product with an RFID tag in astore or another nearby store.

The RFID tag location system can also intelligently route stock requeststo a particular employee or location to minimize time to deliverrequested stock to a particular area of a store or to a customer.Internal stock requests may be routed to employees based on theirproximity to the area of the store, a customer, or to the stockroom andability to complete task in the shortest time. For example, employee Ais working on 5 stock requests for other customers, they must prioritizecompleting those 5 stock requests. The RFID tag location system can thenroute additional requests to the nearest available employee, e.g.,employee B, to fulfill the stock request. Employees may also have theoption to turn off or mute notifications for stock requests if currentlyperforming an unrelated task. The RFID tag location system can alsomonitor the time taken for an employee to complete a stock request, fromthe initial receipt of the stock request to delivery of product to acustomer or area of the store, by tracking the product and employee asthey move through the store.

For external stock requests, the RFID tag location system can activelymonitor and update the availability of products at multiple stores. Forexample, if a customer at second store has a requested product in theirshopping cart, the RFID tag location system will remove this productfrom the available stock at the second store to ensure customers at thefirst store have accurate information regarding product availability.The RFID tag location system can also be used to predict the timerequired for an externally requested product to be delivered to a storeor customer's preferred address based on the distance between theoriginating location, e.g., a second store or warehouse, and thedestination and data detailing the speed that stock requests arefulfilled and shipped by the originating location.

11.15 Stock Request Fulfillment

The RFID tag location system actively tracks the location of productsavailable on a sales floor of a store and a stockroom in real-time andat all times. This active tracking can facilitate employees to quicklycomplete stock requests to customers. For example, a customer mayrequest a stock level check for a particular product in the store usingthe User App. FIG. 11 shows an example GUI where a customer isrequesting a product with options to specify a particular productvariant, e.g., a medium-sized, black dress, from a stockroom to bedelivered to them on the sales floor. This request can then be sent toan employee on the sales floor. The sales floor employee can then usethe User App to request the customer requested product from a stockroomemployee. The stockroom employee can then locate and pick up multiplerequested products for different customers. To facilitate delivery ofthe products to the different customers on the sales floor, thestockroom employee can use the User App, which actively monitors thelocation of the different customers in real-time.

In addition to stock requests, there may be cases where a product ismisplaced or not easily found by a customer but are nonetheless presenton the sales floor. The User App can provide the location of theproducts, if present, in various areas of the store. For example, inFIG. 12, the GUI may show a customer the locations of a selected producton the sales floor, in addition to the quantity available in astockroom. In the event the employees of the store do not move themisplaced products on the sales floor, the User App can also enable acustomer to locate said misplaced products.

11.16 Automatic Marking of a Pick List

A pick list refers to a list of products requested by a user and mayinclude requests for internal replenishment, e-commerce orders, stockrequests, misplaced products, or any other list of products a user needsto locate. Products in the pick list can be automatically marked aspicked up if the following conditions are met: (1) the user is workingon a pick list using the User App, (2) the user picks up a product inthe pick list, and (3) the RFID/computer vision item location systemrecognizes a product on the user's pick list is picked up by the user ifthe product is moving with the user based on the user's device or theirRFID employee tag. Once these conditions are met, the RFID/computervision item location system should automatically mark the product aspicked up by the user. To improve the accuracy of automatic marking ofpick lists, a threshold may be used to determine if a product is pickedup by a user such as time after product is picked up or distance theproduct has moved.

11.17 Tracking Customers With High Cart Values

The RFID tag location system can also be used to actively track thequantity and type of products in a customer's shopping cart inreal-time. A shopping cart may include a basket, a bag, a cart, etc. Inthe event a customer's shopping cart contains products that exceed auser-defined threshold, e.g., 5 total units or $500 value, anotification can be automatically sent to employees identifying thesecustomers. Additionally, specific products or product categories canalso be flagged by employees to prioritize tracking. This trackingfeature may have multiple functional uses in a store. For example, thetracking feature can be used to prevent shoplifting by trackingcustomers who may have a large quantity or value of products in theirshopping cart or may have selected numerous flagged products. Thetracking feature can also be used to identify customers who may bewilling to spend more money, which can notify employees to provide thesecustomers with better customer service, to upsell these customers, or torecommend complimentary products to these customers.

The RFID tag location data may be displayed to users using the GUI inthe User App in various formats. For example, employees can view allcustomers within a store in the GUI and monitor their shopping cartsbased on the quantity of products or by the total value. To facilitateidentification of a customer, if a customer uses a customer variant ofthe User App, the RFID tag location system can associate a person in thestore with a customer profile. Otherwise, a customer may be still beidentified with products in their shopping cart by tracking the motionof products and determining whether the products are associated with aregistered employee device.

11.18 Automatic Notification of VIP Customers

The RFID tag location system may also store data on customers. This datamay include the number of visits a customer makes to a store or theamount of money a customer spends on a monthly or yearly basis. Based onthis data, a VIP designation can be attributed to customers who exceeduser-defined thresholds.

The RFID tag location system can then be used to detect and identify aVIP customer and to notify employees when a VIP enters a store or aparticular section of a store. Identification of a VIP may beaccomplished using numerous methods including (1) detection of a VIPstatus based on a customer's profile stored in the User App on acustomer's mobile device via Bluetooth or Wi-Fi, (2) recognition of acustomer mobile device id based on the User App on a customer's mobiledevice, or (3) identification based on facial or gait recognition usingthe computer vision capabilities of the RFID tag location system.

11.19 Identification of Potential Product Theft

As described earlier, the RFID tag location system can actively trackthe movement of products in a customer's shopping cart in real-time.Potential theft of products can be detected if anomalous events occurwhile monitoring the products. For example, if a customer were to removea RFID tag from a product, the RFID tag location system can detect thisremoval and immediately notify an employee this exact product and itslast known location in the store. Furthermore, the RFID tag locationsystem can also identify and retrieve video footage recorded by thesystem's cameras or RFID readers to assist employees in locating thecustomer or product. Once this information is provided to an employee,the employee can then approach the customer to offer assistance with theproduct missing the RFID tag.

The RFID tag location system can also timestamp and store anomalousevents related to RFID tagged products. This information can be used toshow employees in the GUI potentially high theft zones within the storebased on data such as the frequency of RFID tags that disappear. Thisdata can be viewed in the GUI with user-defined time frames, e.g.,previous 7, 30, 180 days, and so on. Furthermore, the RFID tag locationsystem can also identify and highlight zones in the store that maypotentially become high theft zones by identifying the current locationof RFID tags that are prone to disappearing within the store.

11.20 Automatic Monitoring of Fitting Rooms

A detection strategy similar to the identification of potential producttheft can also be used for automated monitoring of fitting rooms. TheRFID tag location system can track products as they enter or exit afitting room. Notifications can be sent to employees of products thatenter or exit a fitting room in real-time. In the event a product isleft in a fitting room, the RFID tag location system can notify theemployee a stray product is present in the fitting room and to returnthe product to its correct location within the store. If a RFID tag wereto be removed, resulting in the disappearance of a product in the RFIDtag location system, notifications can also be sent to employees that aproduct may have disappeared and identify the customer the product waslast associated with. A customer may also be identified by otherproducts in their shopping cart when the product in questiondisappeared.

11.21 Capture and Measure of Customer and Product Interaction

The RFID tag location system can also be used to detect and measure datarelated to customer and product interaction within a store. For example,the system may track (1) how frequently a product is picked up by acustomer, (2) how long a product is viewed by a customer, (3) whatproducts are viewed together, (4) what product a customer is holdingwhile viewing a new product, (5) what products may be taken from afitting room, (6) what products a customer interacts with prior torealizing purchase, (7) products that may be tested by a customer, e.g.,a customer trying on clothing, based on measured distortion of a RFsignal due to proximity to a body of water, e.g., the human body, (8)how long a customer tests a product. For products tested by a customer,information may also be collected on unpurchased products, e.g.,clothing left in a fitting room, to assess manufacturing or fit issues,e.g., customers prefer the look of a piece of clothing, but not its fit.This data can be used to inform stores how to potentially modifymanufacture of the products to improve sales.

To measure these parameters, the RFID tag location system is capable oftracking objects in a 3D space with high spatial and temporalresolution. For example, the RFID tag location system can detect if aproduct is moved beyond a threshold distance, e.g., 4 inches, and heldfor threshold period of time, e.g., more than 3 seconds. If suchconditions are met, the product can be considered as picked up or viewedby a customer.

As described earlier, the RFID tag location system can track the motionof products and customers within a store. Customers may be identifiedeither by (1) a customer using a User App on their mobile device, whichis detected by Bluetooth, Wi-Fi, or another wireless communicationsystem or sensor, or (2) by detecting customers based on individuals whoneither have a device or tag identifiable by the RFID tag locationsystem assuming employees will have a tag or device.

The RFID tag location system can also collect product performance dataaccording to product groups such as product categories, sub-categories,products, colors, sizes, price ranges, any combination of the formertypes described, and so on. For these product groups, the performancedata that can be collected include the following: (1) the most or leastviewed product groups, (2) the longest or shortest viewed productgroups, (3) product groups that are taken to a fitting the most orleast, (4) product groups that are tested or tried on the most or least,(5) product groups that are tested or tried on the longest or shortestperiod of time, (6) product groups that have the best or worstconversion, which is defined as quantity of sales versus the other typesof data mentioned. For example, if a product is viewed 100 times dailyand sold 10 times daily, the conversion rate is 10%. In another example,if 100 products are tried on for more than 30 seconds and have 10 sales,then products that are tried on for more than 30 seconds have a 10%conversion rate.

Based on the product performance data collected by the RFID tag locationsystem, improvements to store operation can be achieved by: (1)identifying best performing sales areas in the store, (2) identifyingareas with the most product interaction to improve staffing in thatarea, or (3) merchandising strategies such as automatic calculation andrecommendation of product assortments based on best-sellingcombinations, e.g., black jeans and white T-shirts perform besttogether, or identifying areas of a store that are best for certainproduct types, e.g., dresses have the highest conversion rate in Zone Aof a store.

Similarly, the product performance data can improve the consumershopping experience by: (1) understanding a customer's historicalshopping preferences, based on the product groups defined earlier, tonotify customer's the arrival of new or restocked products the customerwas previously searching in the store or online, (2) personalizing thein-store shopping experience by highlighting products within a store orwithin a zone in the store that a customer may be interested in, (3)notifying customers of potential in-store promotions, or (4) identifyingthe customer by detecting a customer's profile stored in the User App ona customer's mobile device via Bluetooth or WiFi, recognizing a customermobile device id based on the User App on a customer's mobile device, oridentifying a customer based on facial or gait recognition using thecomputer vision capabilities of the RFID tag location system.

12 Conclusion

While various embodiments have been described and illustrated herein, avariety of other means and/or structures for performing the functionand/or obtaining the results and/or one or more of the advantagesdescribed herein is possible. More generally, all parameters,dimensions, materials, and configurations described herein are meant tobe exemplary and that the actual parameters, dimensions, materials,and/or configurations will depend upon the specific application orapplications for which the disclosed teachings is/are used. It is to beunderstood that the foregoing embodiments are presented by way ofexample only and that embodiments may be practiced otherwise than asspecifically described and claimed. Embodiments of the presentdisclosure are directed to each individual feature, system, article,material, kit, and/or method described herein. In addition, anycombination of two or more such features, systems, articles, materials,kits, and/or methods, if such features, systems, articles, materials,kits, and/or methods are not mutually inconsistent, is included withinthe scope of the present disclosure.

The above-described embodiments can be implemented in any of numerousways. For example, embodiments of the technology disclosed herein may beimplemented using hardware, software or a combination thereof. Whenimplemented in software, the software code can be executed on anysuitable processor or collection of processors, whether provided in asingle computer or distributed among multiple computers.

The various methods or processes outlined herein may be coded assoftware that is executable on one or more processors that employ anyone of a variety of operating systems or platforms. Additionally, suchsoftware may be written using any of a number of suitable programminglanguages and/or programming or scripting tools, and also may becompiled as executable machine language code or intermediate code thatis executed on a framework or virtual machine.

In this respect, various inventive concepts may be embodied as acomputer readable storage medium (or multiple computer readable storagemedia) (e.g., a computer memory, one or more floppy discs, compactdiscs, optical discs, magnetic tapes, flash memories, circuitconfigurations in Field Programmable Gate Arrays or other semiconductordevices, or other non-transitory medium or tangible computer storagemedium) encoded with one or more programs that, when executed on one ormore computers or other processors, perform methods that implement thevarious embodiments of the invention discussed above. The computerreadable medium or media can be transportable, such that the program orprograms stored thereon can be loaded onto one or more differentcomputers or other processors to implement various aspects of thepresent invention as discussed above.

The terms “program” or “software” are used herein in a generic sense torefer to any type of computer code or set of computer-executableinstructions that can be employed to program a computer or otherprocessor to implement various aspects of embodiments as discussedabove. Additionally, it should be appreciated that according to oneaspect, one or more computer programs that when executed perform methodsof the present invention need not reside on a single computer orprocessor, but may be distributed in a modular fashion amongst a numberof different computers or processors to implement various aspects of thepresent invention.

Computer-executable instructions may be in many forms, such as programmodules, executed by one or more computers or other devices. Generally,program modules include routines, programs, objects, components, datastructures, etc. that perform particular tasks or implement particularabstract data types. Typically, the functionality of the program modulesmay be combined or distributed as desired in various embodiments.

Also, various disclosed concepts may be embodied as one or more methods,of which an example has been provided. The acts performed as part of themethod may be ordered in any suitable way. Accordingly, embodiments maybe constructed in which acts are performed in an order different thanillustrated, which may include performing some acts simultaneously, eventhough shown as sequential acts in illustrative embodiments.

All definitions, as defined and used herein, should be understood tocontrol over dictionary definitions, definitions in documentsincorporated by reference, and/or ordinary meanings of the definedterms.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases. Multiple elements listed with“and/or” should be construed in the same fashion, i.e., “one or more” ofthe elements so conjoined. Other elements may optionally be presentother than the elements specifically identified by the “and/or” clause,whether related or unrelated to those elements specifically identified.Thus, as a non-limiting example, a reference to “A and/or B”, when usedin conjunction with open-ended language such as “comprising” can refer,in one embodiment, to A only (optionally including elements other thanB); in another embodiment, to B only (optionally including elementsother than A); in yet another embodiment, to both A and B (optionallyincluding other elements); etc.

As used herein in the specification and in the claims, “or” should beunderstood to have the same meaning as “and/or” as defined above. Forexample, when separating items in a list, “or” or “and/or” shall beinterpreted as being inclusive, i.e., the inclusion of at least one, butalso including more than one, of a number or list of elements, and,optionally, additional unlisted items. Only terms clearly indicated tothe contrary, such as “only one of” or “exactly one of,” or, when usedin the claims, “consisting of,” will refer to the inclusion of exactlyone element of a number or list of elements. In general, the term “or”as used herein shall only be interpreted as indicating exclusivealternatives (i.e. “one or the other but not both”) when preceded byterms of exclusivity, such as “either,” “one of,” “only one of” or“exactly one of.” “Consisting essentially of,” when used in the claims,shall have its ordinary meaning as used in the field of patent law.

As used herein, the terms “about” and “approximately” generally meanplus or minus 10% of the value stated.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) canrefer, in one embodiment, to at least one, optionally including morethan one, A, with no B present (and optionally including elements otherthan B); in another embodiment, to at least one, optionally includingmore than one, B, with no A present (and optionally including elementsother than A); in yet another embodiment, to at least one, optionallyincluding more than one, A, and at least one, optionally including morethan one, B (and optionally including other elements); etc.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” “composed of,” and the like are tobe understood to be open-ended, i.e., to mean including but not limitedto. Only the transitional phrases “consisting of” and “consistingessentially of” shall be closed or semi-closed transitional phrases,respectively, as set forth in the United States Patent Office Manual ofPatent Examining Procedures, Section 2111.03.

1. A method of locating a radio-frequency identification (RFID) tag, themethod comprising: receiving, with an antenna array, a reference RFIDsignal from a reference RFID tag in a known location; determining areceptivity pattern of the antenna array based on the reference RFIDsignal; receiving, with the antenna array, an RFID signal from an RFIDtag in an unknown location; and determining a location of the RFID tagbased on the RFID signal and the receptivity pattern of the antennaarray.
 2. The method of claim 1, wherein determining the location of theRFID tag comprises: de-convolving, or otherwise correcting, the RFIDsignal and the receptivity pattern of the antenna array to yield a trueRFID signal, the true RFID signal comprising a plurality of peaks;identifying a first peak in the plurality of peaks as corresponding to aline-of-sight (LOS) path between the RFID tag and the antenna array; andestimating the location of the RFID tag based on at least one of alength or an angle of arrival associated with the LOS path.
 3. Themethod of claim 2, wherein identifying the first peak in the pluralityof peaks as corresponding to the LOS path comprises distinguishing thefirst peak from peaks corresponding to non-line-of-sight (NLOS) pathsbetween the RFID tag and the antenna array.
 4. The method of claim 3,wherein distinguishing the first peak from peaks corresponding to theNLOS paths is based on at least one of a slope or a height of the firstpeak.
 5. A system for locating a radio-frequency identification (RFID)tag, the system comprising: an antenna array configured to receive areference RFID signal from at least one reference RFID tag and toreceive an RFID signal from an RFID tag in an unknown location; aprocessor, operably coupled to the antenna array, to determine areceptivity pattern of the antenna array based on the reference RFIDsignal and to determine a location of the RFID tag based on the RFIDsignal and the receptivity pattern of the antenna array.
 6. The systemof claim 5, wherein the processor is configured to determine thelocation of the RFID tag by: de-convolving, or otherwise correcting, theRFID signal and the receptivity pattern of the antenna array to yield atrue RFID signal, the true RFID signal comprising a plurality of peaks;identifying a first peak in the plurality of peaks as corresponding to aline-of-sight (LOS) path between the RFID tag and the antenna array; andestimating the location of the RFID tag based on at least one of alength or an angle of arrival associated with the LOS path.
 7. Thesystem of claim 6, wherein the processor is configured to identify thefirst peak in the plurality of peaks as corresponding to the LOS path bydistinguishing the first peak from peaks corresponding tonon-line-of-sight (NLOS) paths between the RFID tag and the antennaarray.
 8. The system of claim 7, wherein the processor is configured todistinguish the first peak from peaks corresponding to the NLOS pathsbased on at least one of a slope or a height of the first peak.
 9. Amethod of locating a radio-frequency identification (RFID) tag, themethod comprising: receiving, with a first antenna at a first locationand a second antenna at a second location different than the firstlocation, reference RFID signals from respective reference RFID tags atdifferent angles of arrival, the reference RFID tags being at respectiveknown locations; calibrating receptivity patterns of the first antennaand the second antenna based on the different angles of arrival of thereference RFID signals and the known locations of the reference RFIDtags; receiving, with the first antenna and the second antenna, an RFIDsignal from an RFID tag at an unknown location; determining angles ofarrival of the RFID signal to the first antenna and the second antennabased on the receptivity patterns of the first antenna and the secondantenna; and determining an estimated location of the RFID tag based onthe angles of arrivals.
 10. The method of claim 9, wherein receiving theRFID signal at the first antenna comprises: detecting a multipathprofile comprising a first peak associated with a line-of-sight (LOS)path between the RFID tag and the first antenna and a second peakassociated with a non-line-of-sight (NLOS) path between the RFID tag andthe first antenna.
 11. The method of claim 10, wherein determining theangles of arrival comprises: identifying the first peak as associatedwith the LOS path based on at least one of an amplitude and a slope ofthe first peak with respect to at least one of an amplitude and a slopeof the second peak.
 12. The method of claim 9, further comprising:receiving, with a third antenna at a third location different than thefirst location and the second location, reference RFID signals from therespective reference RFID tags at different angles of arrival betweenthe third antenna and the respective reference RFID tags; calibrating areceptivity pattern of the third antenna based on the different anglesof arrival between the third antenna and the respective reference RFIDtags and the known locations of the reference RFID tags; receiving, withthe third antenna, the RFID signal from the RFID tag; and determining anangle of arrival of the RFID signal to the third antenna based on thereceptivity pattern of the third antenna, and wherein determining theestimated location of the RFID tag is further based on the angle ofarrival of the RFID signal to the third antenna.
 13. A system forlocating a radio-frequency identification (RFID) tag, the systemcomprising: a first antenna at a first location and a second antenna ata second location different than the first location to receive referenceRFID signals from respective reference RFID tags at different angles ofarrival, the reference RFID tags being at respective known locations,and to receive an RFID signal from an RFID tag at an unknown location;and a processor, operably coupled to the first antenna and to the secondantenna, to calibrate receptivity patterns of the first antenna and thesecond antenna based on the different angles of arrival of the referenceRFID signals and the known locations of the reference RFID tags, todetermine angles of arrival of the RFID signal to the first antenna andthe second antenna based on the receptivity patterns of the firstantenna and the second antenna, and to determine an estimated locationof the RFID tag based on the angles of arrivals.
 14. The system of claim13, wherein the first antenna is configured to detect a multipathprofile comprising a first peak associated with a line-of-sight (LOS)path between the RFID tag and the first antenna and a second peakassociated with a non-line-of-sight (NLOS) path between the RFID tag andthe first antenna.
 15. The system of claim 14, wherein the processor isconfigured to identify the first peak as associated with the LOS pathbased on at least one of an amplitude and a slope of the first peak withrespect to at least one of an amplitude and a slope of the second peak.16. The system of claim 13, further comprising: a third antenna at athird location different than the first location and the second locationto receive reference RFID signals from the respective reference RFIDtags at different angles of arrival between the third antenna and therespective reference RFID tags and to receive the RFID signal from theRFID tag, and wherein the processor is configured to calibrate areceptivity pattern of the third antenna based on the different anglesof arrival between the third antenna and the respective reference RFIDtags and the known locations of the reference RFID tags, to determine anangle of arrival of the RFID signal to the third antenna based on thereceptivity pattern of the third antenna, and to determine the estimatedlocation of the RFID tag based on the angle of arrival of the RFIDsignal to the third antenna.
 17. The system of claim 13, wherein theprocessor is configured to transmit, to a mobile device, the estimatedposition of the RFID tag.
 18. The system of claim 17, wherein theestimated position is in at least one of a store, a stockroom, or awarehouse, and the mobile device is configured to display a path to theRFID tag.
 19. The system of claim 18, wherein the mobile device isconfigured to display a path from the RFID tag to a target location forthe RFID tag.
 20. The system of claim 19, wherein the RFID tag is afirst RFID tag, and wherein the mobile device is further configured todisplay a path from the first RFID tag to an estimated position of asecond RFID tag en route to the target location.
 21. The system of claim17, wherein the mobile device is further configured to displayinformation about an item associated with the RFID tag.